Semiconductor device

ABSTRACT

A semiconductor device includes first and second transistors having the same conductivity type and a circuit. One of a source and a drain of the first transistor is electrically connected to that of the second transistor. First and third potentials are supplied to the circuit through respective wirings. A second potential and a first clock signal are supplied to the others of the sources and the drains of the first and second transistors, respectively. A second clock signal is supplied to the circuit. The third potential is higher than the second potential which is higher than the first potential. A fourth potential is equal to or higher than the third potential. The first clock signal alternates the second and fourth potentials and the second clock signal alternates the first and third potentials. The circuit controls electrical connections between gates of the first and second transistors and the wirings.

BACKGROUND OF THE INVENTION

1. Field of the Invention

One embodiment of the present invention relates to a semiconductordevice. In particular, one embodiment of the present invention relatesto a semiconductor device such as a sequential circuit that includestransistors having the same conductivity type and a semiconductordisplay device that includes the sequential circuit.

2. Description of the Related Art

A semiconductor display device in which a driver circuit is constitutedof transistors having the same conductivity type is preferable becausethe manufacturing cost can be lowered. Patent Documents 1 and 2 disclosetechniques for forming a variety of circuits such as inverters and shiftregisters that are used in driver circuits of semiconductor displaydevices and are constituted of transistors having the same conductivitytype.

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    2001-325798-   [Patent Document 2] Japanese Published Patent Application No.    2010-277652

SUMMARY OF THE INVENTION

Transistors having the same conductivity type tend to be normally onbecause the threshold voltages are likely to be shifted in the negativedirection on account of various factors. In a driver circuit of asemiconductor display device constituted of transistors having the sameconductivity type, when the threshold voltages of the transistors areshifted in the negative direction in sequential circuits that outputsignals having pulses, the amplitude of potentials of the output signalsbecomes small and the driver circuit cannot be normally operated;alternatively, even if normal operation is carried out, the powerconsumption of the driver circuit is increased.

For example, in a circuit disclosed in FIG. 10 of Patent Document 2, thepotential of a source of a transistor Q2 is fixed to a low potentialVSS. If the transistor Q2 is normally off, the transistor Q2 is turnedoff when the low potential VSS is supplied to a gate of the transistorQ2. If the transistor Q2 is normally on, even when the low potential VSSis supplied to the gate of the transistor Q2, the voltage (gate voltage)between the gate and the source when the potential of the source is areference is kept higher than the threshold voltage of the transistorQ2. Thus, the transistor Q2 is not turned off but is turned on.

When the transistor Q2 is on though it should be off, wasted currentflows to the circuit, so that consumption current is increased. Further,the wasted current increases current flowing to a wiring for supplying apotential (e.g., in the case of FIG. 10 of Patent Document 2, thelow-level potential VSS and a high-level potential VDD of a clock signalCLKA or the low-level potential VSS) to the circuit. Then, theresistance of the wiring decreases the potential of the wiring suppliedwith the potential VDD and increases the potential of the wiringsupplied with the potential VSS. Accordingly, the amplitude of apotential output from the circuit is smaller than a difference betweenthe potentials VDD and VSS (an ideal potential difference).

In the case where a transistor for controlling electrical connectionbetween a wiring to which a clock signal is supplied and an outputterminal (e.g., the transistor Q1 in FIG. 10 of Patent Document 2) isnormally on, the output terminal is charged and discharged through thetransistor Q1, which increases the power consumption of the circuit.

In particular, in a pixel portion of a semiconductor display device,when a potential output from a circuit is supplied to a wiring called abus line (e.g., a scan line or a signal line) that is connected to aplurality of pixels, a transistor for controlling the output of apotential from the circuit (e.g., the transistors Q1 and Q2 in FIG. 10of Patent Document 2) needs a large current supply capability. Thus, thechannel width W of the transistor is made larger than that of anothertransistor in the circuit in many cases. The drain current of thetransistor is proportional to the channel width W. Thus, in the casewhere the channel width W of a normally-on transistor is made larger,the amount of current flowing to the normally-on transistor is largerthan that of another transistor when the normally-on transistor shouldbe off. Consequently, wasted current flowing to the circuit isincreased, so that the aforementioned increase in power consumption ordecrease in amplitude of a potential output remarkably occurs.

Under the technical background, it is an object of the present inventionto provide a power saving semiconductor device. Alternatively, it is anobject of the present invention to provide a semiconductor devicecapable of preventing a decrease in amplitude of a potential output.

One embodiment of the present invention includes a first transistor forcontrolling the supply of a power supply potential to an outputterminal; a second transistor for controlling the supply of a potentialof a clock signal to an output terminal; and a circuit for controllingelectrical connection between a gate of the first or second transistorand wirings to which a pair of power supply potentials are supplied. Apower supply potential supplied to the output terminal through a sourceand a drain of the first transistor is supplied to a sequential circuitthrough a wiring that is different from the wirings to which the pair ofpower supply potentials are supplied.

With such a configuration, the gate of the first transistor can beelectrically isolated from one of the source and the drain of the firsttransistor. Thus, a power supply potential supplied to the one of thesource and the drain of the first transistor and a power supplypotential supplied to the gate of the first transistor are individuallycontrolled, whereby its gate voltage can be controlled to turn off thefirst transistor. Accordingly, even when the first transistor isnormally on, the first transistor can be turned off when it should beturned off.

In one embodiment of the present invention, in the case where the firstand second transistors are n-channel transistors, one of two levels ofpotentials of the clock signal that is closer to the power supplypotential supplied to the output terminal through the source and thedrain of the first transistor has a potential that is equal to or higherthan the power supply potential. In the case where the first and secondtransistors are p-channel transistors, one of two levels of potentialsof the clock signal that is closer to the power supply potentialsupplied to the output terminal through the source and the drain of thefirst transistor has a potential that is equal to or lower than thepower supply potential.

With such a configuration, even when the second transistor is normallyon, the second transistor can be turned off when it should be turnedoff. Thus, the charge and discharge of the output terminal through thesecond transistor can be prevented, which can keep the power consumptionof the circuit low.

Specifically, a semiconductor device of one embodiment of the presentinvention includes a first wiring to which a first potential (VSS) issupplied; a second wiring to which a second potential (VEE) higher thanthe first potential is supplied; a third wiring to which a thirdpotential (VDD) higher than the second potential is supplied; a fourthwiring to which a first clock signal (CLKB) in which a fourth potential(VCC) that is equal to or higher than the third potential and the secondpotential are alternated is supplied; a first transistor and a secondtransistor having the same conductivity type; and a circuit forcontrolling electrical connection between a gate of the first or secondtransistor and the first or third wiring in accordance with a secondclock signal in which the first potential and the third potential arealternated and an input signal (Vin). One of a source and a drain of thefirst transistor is electrically connected to the second wiring. One ofa source and a drain of the second transistor is electrically connectedto the fourth wiring. The other of the source and the drain of the firsttransistor is electrically connected to the other of the source and thedrain of the second transistor.

In one embodiment of the present invention, it is possible to provide apower saving semiconductor device constituted of transistors having thesame conductivity type. Alternatively, in one embodiment of the presentinvention, it is possible to provide a semiconductor device capable ofpreventing a decrease in amplitude of a potential output.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates a configuration of a sequential circuit and FIG. 1Bshows waveforms of potentials of clock signals.

FIG. 2 illustrates a configuration of a sequential circuit.

FIG. 3 is a timing diagram showing operation of a sequential circuit.

FIG. 4 illustrates a configuration of a shift register.

FIG. 5 is a timing diagram of operation of a shift register.

FIG. 6 schematically illustrates a j-th sequential circuit 10 _(—) j.

FIGS. 7A and 7B each illustrate a configuration of a sequential circuit.

FIGS. 8A and 8B each illustrate a configuration of a sequential circuit.

FIG. 9 illustrates a configuration of a sequential circuit.

FIGS. 10A to 10C illustrate configurations of a semiconductor displaydevice.

FIG. 11 is a top view of a pixel.

FIG. 12 is a cross-sectional view of the pixel.

FIGS. 13A and 13B are cross-sectional views illustrating a structure ofa transistor.

FIG. 14 is a top view of a liquid crystal display device.

FIG. 15 is a cross-sectional view of a liquid crystal display device.

FIGS. 16A to 16F each illustrate an electronic device.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention are described below in detail withreference to the drawings. Note that the present invention is notlimited to the following description, and it is easily understood bythose skilled in the art that the mode and details can be variouslychanged without departing from the spirit and scope of the presentinvention. Therefore, the present invention should not be construed asbeing limited to the description of the embodiments below.

The present invention encompasses in its category, any semiconductordevice using a transistor, such as an integrated circuit, an RF tag, anda semiconductor display device. The integrated circuits include, in itscategory, large scale integrated circuits (LSIs) including amicroprocessor, an image processing circuit, a digital signal processor(DSP), a microcontroller, and the like, and programmable logic devices(PLDs) such as a field programmable gate array (FPGA) and a complex PLD(CPLD). Further, the semiconductor display device includes, in itscategory, semiconductor display devices in which circuit elementsincluding semiconductor films are included in driver circuits, such asliquid crystal display devices, light-emitting devices in which alight-emitting element typified by an organic light-emitting element isprovided in each pixel, electronic paper, digital micromirror devices(DMDs), plasma display panels (PDPs), and field emission displays(FEDs).

In this specification, the semiconductor display device includes in itscategory, panels in which a display element such as a liquid crystalelement or a light-emitting element is provided for each pixel, andmodules in which an IC or the like including a controller is mounted onthe panel.

Note that “connection” in this specification means electrical connectionand corresponds to the state in which current, voltage, or a potentialcan be supplied or transmitted. Accordingly, a connection state meansnot only a state of direct connection but also a state of electricalconnection through a circuit element such as a wiring, a resistor, adiode, or a transistor so that current, voltage, or a potential can besupplied or transmitted.

Note that a “source” of a transistor means a source region that is partof a semiconductor film functioning as an active layer or a sourceelectrode electrically connected to the semiconductor film. Similarly, a“drain” of a transistor means a drain region that is part of asemiconductor film functioning as an active layer or a drain electrodeelectrically connected to the semiconductor film. A “gate” means a gateelectrode.

The terms “source” and “drain” of a transistor interchange with eachother depending on the type of the channel of the transistor or levelsof potentials supplied to the terminals. In general, in an n-channeltransistor, a terminal to which a lower potential is supplied is calleda source, and a terminal to which a higher potential is supplied iscalled a drain. Further, in a p-channel transistor, a terminal to whicha lower potential is supplied is called a drain, and a terminal to whicha higher potential is supplied is called a source. In thisspecification, although connection relation of the transistor isdescribed assuming that the source and the drain are fixed in some casesfor convenience, actually, the names of the source and the draininterchange with each other depending on the relation of the potentials.

Configuration Examples of Sequential Circuit

FIG. 1A illustrates an example of the configuration of a sequentialcircuit in one embodiment of the present invention. A sequential circuit10 in FIG. 1A includes a circuit 11 including a plurality oftransistors, a transistor 12, and a transistor 13. In the sequentialcircuit 10 in FIG. 1A, at least the transistors 12 and 13 have the sameconductivity type. In FIG. 1A, the transistors 12 and 13 are n-channeltransistors.

A high-level power supply potential VDD is supplied to the circuit 11through a wiring 14. A low-level power supply potential VSS is suppliedto the circuit 11 through a wiring 15. A potential of an input signalVin is supplied to the circuit 11 through a wiring 17. Potentials of aplurality of clock signals CLKA are supplied to the circuit 11 through aplurality of wirings 18.

The circuit 11 has a function of controlling electrical connectionbetween a gate of the transistor 12 or 13 and the wiring 14 or 15 inaccordance with the potential of the input signal Vin and the pluralityof clock signals CLKA.

The transistor 12 has a function of controlling electrical connectionbetween a wiring 16 to which a low-level power supply potential VEE issupplied and an output terminal 20. The transistor 13 has a function ofcontrolling electrical connection between a wiring 19 to which a clocksignal CLKB is supplied and the output terminal 20.

Specifically, one of a source and a drain of the transistor 12 iselectrically connected to the wiring 16. The other of the source and thedrain of the transistor 12 is electrically connected to the outputterminal 20. One of a source and a drain of the transistor 13 iselectrically connected to the wiring 19. The other of the source and thedrain of the transistor 13 is electrically connected to the outputterminal 20.

A capacitor having a function of holding a gate voltage of thetransistor 13 may be connected to the gate of the transistor 13. Notethat in the case where the gate voltage of the transistor 13 can be heldwithout provision of the capacitor, for example, in the case where theparasitic capacitance of the gate of the transistor 13 is high, thecapacitor is not necessarily provided.

When a potential Vout output from the output terminal 20 of thesequential circuit 10 is supplied to a wiring called a bus line that isconnected to a plurality of pixels, the transistors 12 and 13 forcontrolling the output of the potential Vout needs a large currentsupply capability. Thus, the channel widths W of the transistors 12 and13 are preferably greater than those of transistors in the circuit 11.

The power supply potential VEE is preferably lower than the power supplypotential VDD and higher than the power supply potential VSS.

In one embodiment of the present invention, two levels of potentials arealternated in the clock signal CLKB, and a low-level potential of thetwo levels of potentials that is closer to the power supply potentialVSS is equal to or higher than the power supply potential VEE. FIG. 1Bexemplifies a waveform of a potential of a clock signal CLKA in whichthe power supply potential VSS and the power supply potential VDD arealternated and a waveform of a potential of a clock signal CLKB in whichthe power supply potential VEE and a power supply potential VCC higherthan the power supply potential VDD are alternated. Although FIG. 1Bexemplifies the case where the high-level potential of the clock signalCLKB is the power supply potential VCC that is higher than the powersupply potential VDD, a potential that is equal to or higher than thepower supply potential VDD is acceptable as the high-level potential ofthe clock signal CLKB.

In the case where the transistor 13 is an n-channel transistor, when apotential higher than the power supply potential VDD is supplied fromthe circuit 11 to the gate of the transistor 13, the high-levelpotential VCC of the clock signal CLKB that is supplied to the one ofthe source and the drain of the transistor 13 is supplied to the outputterminal 20 through the transistor 13 in an on-state. Then, the powersupply potential VSS is supplied from the circuit 11 to the gate of thetransistor 13, and the low-level potential VEE of the clock signal CLKBis supplied to the one of the source and the drain of the transistor 13,so that the gate voltage of the transistor 13 is a potential VSS−VEE.Even if the transistor 13 is normally on, the transistor 13 can beturned off by setting the level of the potential VEE to satisfyVSS−VEE≦Vth; thus, an increase in power consumption due to charge anddischarge of the wiring 18 through the transistor 13 can be prevented.

In the case where the transistor 12 is an n-channel transistor, thetransistor 12 is turned on when the power supply potential VDD or apotential lower than the power supply potential VDD by the thresholdvoltage of the transistors provided in the circuit 11 is supplied fromthe circuit 11 to the gate of the transistor 12. When the power supplypotential VSS is supplied from the circuit 11 to the gate of thetransistor 12, a gate voltage Vgs is VSS−VEE. Even if the transistor 12is normally on, the transistor 12 can be turned off by setting the levelof the potential VEE to satisfy VSS−VEE≦Vth; thus, an increase in powerconsumption can be prevented.

When the transistors in the circuit 11 are normally on as well as thetransistor 12, current flows in the wiring 15 through the transistors inthe circuit 11, which raises the potential of the wiring 15. Since thepotential of the wiring 15 is supplied to the gate of the transistor 12through the circuit 11, due to the rise in potential of the wiring 15,the potential supplied to the gate of the transistor 12 is also raisedfrom the power supply potential VSS to the potential VSS+Vα.

Even if the potential supplied to the gate of the transistor 12 israised, the transistor 12 is kept off when the following formula issatisfied: Vgs=VSS+Vα−VEE≦Vth. Thus, when the levels of the power supplypotentials VSS and VEE are set to satisfy Vgs≦Vth in anticipation of theamount of an increase in the potential of the wiring 15, even if thetransistor 12 is normally on, the transistor 12 can be turned off whenit should be turned off.

In the case where the potential Vout output from the output terminal 20in the sequential circuit 10 is supplied to the wiring called a bus linethat is connected to the plurality of pixels, a large current supplycapability is needed for the transistors 12 and 13, and in contrast, thecurrent supply capability of the transistors in the circuit 11 does nothave to be as large as that of the transistors 12 and 13. Therefore, thechannel widths W of the transistors in the circuit 11 can be smallerthan those of the transistors 12 and 13. Accordingly, even if thetransistors in the circuit 11 is normally on and the gate voltage issomewhat greater than the threshold voltage, current flowing in thewiring 15 through the transistors in the circuit 11 can be smaller thanthat flowing in the wiring 16 when the gate voltage of the transistor 12is somewhat greater than the threshold voltage. Thus, it is relativelyeasy to keep a voltage Vα that corresponds to the amount of change inthe potential of the wiring 15 small.

As described above, in the sequential circuit 10 of one embodiment ofthe present invention, the wiring 16 electrically connected to the oneof the source and the drain of the transistor 12 that is positioned onan output side is electrically isolated from the wiring 15 electricallyconnected to the transistors in the circuit 11, whereby the power supplypotential VEE supplied to the one of the source and the drain of thetransistor 12 and the power supply potential VSS supplied to the gate ofthe transistor 12 can be individually controlled. Accordingly, even whenthe transistor 12 is normally on, the gate voltage of the transistor 12can be controlled so that the transistor 12 can be turned off when itshould be turned off. Thus, the power consumption of the sequentialcircuit 10 can be kept low; furthermore, the amplitude of the potentialVout output from the sequential circuit 10 is prevented from beingsmall.

Note that although the transistors 12 and 13 are n-channel transistorsin FIG. 1A, the transistors 12 and 13 may be p-channel transistors. Notethat in such a case, a potential higher than the wiring 14 is suppliedto the wiring 15 connected to the circuit 11 and supplied to the wiring16 connected to the one of the source and the drain of the transistor12. The potential of the wiring 16 is lower than that of the wiring 15.

Specific Configuration Example 1 of Sequential Circuit

Next, the specific structure example of the sequential circuit 10 isdescribed. FIG. 2 illustrates an example of the configuration of asequential circuit of one embodiment of the present invention.

The sequential circuit 10 in FIG. 2 includes the circuit 11, atransistor 101, and a transistor 102. The transistors 101 and 102correspond to the transistors 12 and 13, respectively, in FIG. 1A. Inthe sequential circuit 10, a variety of power supply potentials aresupplied through wirings 110, 111, and 112, and the clock signals CLKA1,CLKA2, and CLKA3 are supplied through wirings 113, 114, and 115,respectively. The clock signal CLKB is supplied through a wiring 116, aninput signal LIN is supplied through a wiring 117, and an input signalRIN is supplied through a wiring 118. In the sequential circuit 10, anoutput signal SROUT is output through a wiring 119, and an output signalGOUT is output through a wiring 120.

In the sequential circuit 10 in FIG. 2, the circuit 11 includestransistors 130 to 139.

A shift register can be constituted by connecting the sequentialcircuits 10 in the plurality of stages to each other.

In the case where the transistors 101, 102, and 130 to 139 are n-channeltransistors, specifically, the power supply potential VDD is supplied tothe wiring 110, the power supply potential VSS is supplied to the wiring111, and the power supply potential VEE is supplied to the wiring 112.In addition, the input signal LIN is supplied to the wiring 117, and theinput signal RIN is supplied to the wiring 118. The input signal LIN andthe input signal RIN correspond to the input signal Vin of thesequential circuit 10 in FIG. 1A.

A gate of the transistor 130 is connected to gates of the transistors136 and 101. One of a source and a drain of the transistor 130 isconnected to the wiring 111. The other of the source and the drain ofthe transistor 130 is connected to one of a source and a drain of thetransistor 137 and one of a source and a drain of the transistor 139.One of a source and a drain of the transistor 136 is connected to thewiring 111, and the other of the source and the drain of the transistor136 is connected to the wiring 119. One of a source and a drain of thetransistor 101 is connected to the wiring 112, and the other of thesource and the drain of the transistor 101 is connected to the wiring120.

A gate of the transistor 131 is connected to the wiring 117. One of asource and a drain of the transistor 131 is connected to the wiring 110.The other of the source and the drain of the transistor 131 is connectedto the other of the source and the drain of the transistor 130. A gateof the transistor 134 is connected to the wiring 114. One of a sourceand a drain of the transistor 134 is connected to the wiring 110. Theother of the source and the drain of the transistor 134 is connected toone of a source and a drain of the transistor 133. A gate of thetransistor 135 is connected to the wiring 118. One of a source and adrain of the transistor 135 is connected to the wiring 110. The other ofthe source and the drain of the transistor 135 is connected to the gatesof the transistors 130, 136, and 101.

A gate of the transistor 133 is connected to the wiring 115. The otherof the source and the drain of the transistor 133 is connected to thegates of the transistors 130, 136, and 101. A gate of the transistor 132is connected to the wiring 117. One of a source and a drain of thetransistor 132 is connected to the wiring 111. The other of the sourceand the drain of the transistor 132 is connected to the gates of thetransistors 130, 136, and 101. A gate of the transistor 137 is connectedto the wiring 110. The one of the source and the drain of the transistor137 is connected to the other of the source and the drain of thetransistor 131 and the other of the source and the drain of transistor130. The other of the source and the drain of the transistor 137 isconnected to a gate of the transistor 138. One of a source and a drainof the transistor 138 is connected to the wiring 113. The other of thesource and the drain of the transistor 138 is connected to the wiring119.

A gate of the transistor 139 is connected to the wiring 110. The one ofa source and a drain of the transistor 139 is connected to the other ofthe source and the drain of the transistor 131 and the other of thesource and the drain of the transistor 130. The other of the source andthe drain of the transistor 139 is connected to a gate of the transistor102. One of a source and a drain of the transistor 102 is connected tothe wiring 116. The other of the source and the drain of the transistor102 is connected to the wiring 120.

Operation of the sequential circuit 10 illustrated in FIG. 2 isdescribed with reference to a timing diagram in FIG. 3.

As shown in FIG. 3, in Time t1, the potential of the clock signal CLKA1supplied to the wiring 113 is VSS, the potential of the clock signalCLKA2 supplied to the wiring 114 is VDD, the potential of the clocksignal CLKA3 supplied to the wiring 115 is VDD, the potential of theclock signal CLKB supplied to the wiring 116 is VEE, the potential ofthe input signal LIN supplied to the wiring 117 is VSS, and thepotential of the input signal RN supplied to the wiring 118 is VSS.

Thus, in Time t1, the transistors 101, 130, 133, 134, 136, 137, and 139are on, and the transistors 131, 132, 135, 138, and 102 are off in thesequential circuit 10. Accordingly, the power supply potential VEE ofthe wiring 112 is output from the wiring 120 as a potential of theoutput signal GOUT. In addition, the power supply potential VSS of thewiring 111 is output from the wiring 119 as a potential of the outputsignal SROUT.

As shown in FIG. 3, in Time t2, the potential of the clock signal CLKA1supplied to the wiring 113 is VSS, the potential of the clock signalCLKA2 supplied to the wiring 114 is VSS, the potential of the clocksignal CLKA3 supplied to the wiring 115 is VDD, the potential of theclock signal CLKB supplied to the wiring 116 is VEE, the potential ofthe input signal LIN supplied to the wiring 117 is VDD, and thepotential of the input signal RIN supplied to the wiring 118 is VSS.

Thus, in Time t2, the transistors 131, 132, 133, 137, 138, 139, and 102are on, and the transistors 101, 130, 134, 135, and 136 are off in thesequential circuit 10. Accordingly, the potential VEE of the clocksignal CLKB of the wiring 116 is output from the wiring 120 as apotential of the output signal GOUT. In addition, the potential VSS ofthe clock signal CLKA1 of the wiring 113 is output from the wiring 119as a potential of the output signal SROUT.

As shown in FIG. 3, in Time t3, the potential of the clock signal CLKA1supplied to the wiring 113 is VDD, the potential of the clock signalCLKA2 supplied to the wiring 114 is VSS, the potential of the clocksignal CLKA3 supplied to the wiring 115 is VSS, the potential of theclock signal CLKB supplied to the wiring 116 is VCC, the potential ofthe input signal LIN supplied to the wiring 117 is VDD, and thepotential of the input signal RIN supplied to the wiring 118 is VSS.

Thus, in Time t3, the transistors 131, 132, 138, and 102 are on, and thetransistors 101, 130, 133 to 137, and 139 are off in the sequentialcircuit 10. Accordingly, the potential VCC of the clock signal CLKB ofthe wiring 116 is output from the wiring 120 as a potential of theoutput signal GOUT. In addition, the potential VDD of the clock signalCLKA1 of the wiring 113 is output from the wiring 119 as a potential ofthe output signal SROUT.

As shown in FIG. 3, in Time t4, the potential of the clock signal CLKA1supplied to the wiring 113 is VDD, the potential of the clock signalCLKA2 supplied to the wiring 114 is VDD, the potential of the clocksignal CLKA3 supplied to the wiring 115 is VSS, the potential of theclock signal CLKB supplied to the wiring 116 is VCC, the potential ofthe input signal LN supplied to the wiring 117 is VSS, and the potentialof the input signal RN supplied to the wiring 118 is VSS.

Thus, in Time t4, the transistors 134, 138, and 102 are on, and thetransistors 101, 130, 131, 132, 133, 135, 136, 137, and 139 are off inthe sequential circuit 10. Accordingly, the potential VCC of the clocksignal CLKB of the wiring 116 is output from the wiring 120 as apotential of the output signal GOUT. In addition, the potential VDD ofthe clock signal CLKA1 of the wiring 113 is output from the wiring 119as a potential of the output signal SROUT.

As shown in FIG. 3, in Time t5, the potential of the clock signal CLKA1supplied to the wiring 113 is VSS, the potential of the clock signalCLKA2 supplied to the wiring 114 is VDD, the potential of the clocksignal CLKA3 supplied to the wiring 115 is VDD, the potential of theclock signal CLKB supplied to the wiring 116 is VEE, the potential ofthe input signal LN supplied to the wiring 117 is VSS, and the potentialof the input signal RN supplied to the wiring 118 is VDD.

Thus, in Time t5, the transistors 101, 130, 133, 134, 135, 136, 137, and139 are on, and the transistors 131, 132, 138, and 102 are off in thesequential circuit 10. Accordingly, the power supply potential VEE ofthe wiring 112 is output from the wiring 120 as a potential of theoutput signal GOUT. In addition, the power supply potential VSS of thewiring 111 is output from the wiring 119 as a potential of the outputsignal SROUT.

Note that in the above operation, the transistor 101 is off in Times t2to t4. In particular, in Times t3 and t4, since the potential of theclock signal CLKB that is supplied to the wiring 116 is the high-levelpotential VCC, current flows between the wirings 116 and 112 through thetransistors 101 and 102 when the transistor 101 is on. However, in oneembodiment of the present invention, the gate and the one of the sourceand the drain of the transistor 101 are electrically isolated from eachother. Specifically, when the transistor 101 is off, the power supplypotential VSS of the wiring 111 can be supplied to the gate of thetransistor 101, and the power supply potential VEE of the wiring 112 canbe supplied to the one of the source and the drain of the transistor101. Thus, even when current flows between the wirings 116 and 112, thecurrent increases the power supply potential VEE of the wiring 112, andthe gate voltage Vgs of the transistor 101 becomes close to thethreshold voltage Vth. Consequently, the transistor 101 can beeventually turned off.

In the above operation, the transistor 102 is off in Times t1 and t5. InTimes t1 and t5, the power supply potential VSS of the wiring 111 issupplied to the gate of the transistor 102. However, since the potentialVEE of the clock signal CLKB that is higher than the power supplypotential VSS is supplied to the one of the source and the drain of thetransistor 102, the gate voltage of the transistor 102 can be lower thanthe threshold voltage Vth. Specifically, the potential VEE is preferablyhigher than a potential that is obtained by subtracting the thresholdvoltage Vth from the potential VSS.

FIG. 4 illustrates an example of a shift register constituted byconnecting the sequential circuits 10 in the plurality of stages to eachother.

The shift register illustrated in FIG. 4 includes sequential circuits10_1 to 10 _(—) y (y is a natural number). Each of the sequentialcircuits 10_1 to 10 _(—) y has the same structure as the sequentialcircuit 10 illustrated in FIG. 2. Any three of clock signals CLKA1 toCLKA4 are supplied to the wirings 113, 114, and 115 in FIG. 2 as theclock signals CLKA1, CLKA1, and CLKA3, respectively. One of clocksignals CLKB1 to CLKB4 is supplied to the wiring 116 as the clock signalCLKB.

Specifically, in the sequential circuit 10_4 m+1, the clock signalsCLKA1, CLKA2, and CLKA3 are supplied to the wirings 113, 114, and 115,respectively. In the sequential circuit 10_4 m+2, the clock signalsCLKA2, CLKA3, and CLKA4 are supplied to the wirings 113, 114, and 115,respectively. In the sequential circuit 10_4 m+3, the clock signalsCLKA3, CLKA4, and CLKA1 are supplied to the wirings 113, 114, and 115,respectively. In the sequential circuit 10_4 m+4, the clock signalsCLKA4, CLKA1, and CLKA2 are supplied to the wirings 113, 114, and 115,respectively. Note that m is a given integer that meets the conditionthat the total number of the sequential circuits 10 is y.

Specifically, in the sequential circuit 10_4 m+1, the clock signal CLKB1is supplied to the wiring 116. In the sequential circuit 10_4 m+2, theclock signal CLKB4 is supplied to the wiring 116. In the sequentialcircuit 10_4 m+3, the clock signal CLKB3 is supplied to the wiring 116.In the sequential circuit 10_4 m+4, the clock signal CLKB2 is suppliedto the wiring 116.

FIG. 6 schematically illustrates the positions of the wirings 113 to 120in the sequential circuit 10 _(—) j (j is a natural number equal to orsmaller than y) in the shift register in FIG. 4. As seen from FIG. 4 andFIG. 6, the output signal SROUTj−1 that is output from the wiring 119 inthe sequential circuit 10 _(—) j−1 in the previous stage is supplied tothe wiring 117 of the sequential circuit 10 _(—) j as the input signalLN. Note that a potential of a start pulse signal SP is supplied to thewiring 117 in the sequential circuit 10_1 in the first stage.

The output signal SROUTj+2 that is output from the wiring 19 in thesequential circuit 10 _(—) j+2 in the stage following the next stage issupplied to the wiring 118 in the sequential circuit 10 _(—) j as theinput signal RN. Note that the input signal RIN_y−1 is supplied to thewiring 118 in the sequential circuit 10 _(—) y−1 in the (y−1)-th stage,and the input signal RIN_y is supplied to the wiring 118 in thesequential circuit 10 _(—) y in the y-th stage. The input signal RIN_y−1is the output signal SROUTy+1 that may be output from the sequentialcircuit 10 _(—) y+1 assuming that the sequential circuit 10 _(—) y+1 ispresent. Further, the input signal RIN_y is the output signal SROUTy+2that may be output from the sequential circuit 10 _(—) y+2 assuming thatthe sequential circuit 10 _(—) y+2 is present.

The output signal GOUT j is output from the wiring 120 in the sequentialcircuit 10 _(—) j.

FIG. 5 is a timing diagram of the potentials of the clock signals CLK1to CLK4, the potential of the start pulse signal SP, and the potentialsof the output signals GOUT 1 to GOUT 3. The clock signals CLK1 to CLK4have waveforms whose potential rise timings are shifted backward by ¼period. The shift register illustrated in FIG. 4 operates in response tothe signals. The shift register illustrated in FIG. 4 outputs the outputsignals GOUT 1 to GOUT y having pulse widths which correspond to ½period of the clock signals and waveforms whose pulses are shiftedbackward by ¼ period of the clock signals.

For example, in the case where the output signals GOUT 1 to GOUT y aresupplied to a wiring called a bus line that is connected to a pluralityof pixels in a semiconductor display device by the shift registerillustrated in FIG. 4, the output-side transistors 101 and 102 in eachof the sequential circuits 10_1 to 10 _(—) y need to have a largecurrent supply capability. Thus, the channel width W of each of thetransistors 101 and 102 is made larger than that of a transistor otherthan the transistors 101 and 102 in many cases. Consequently, when thetransistors 101 and 102 are normally on, an increase in powerconsumption of the shift register or a decrease in amplitude of theoutput signals GOUT 1 to GOUT y remarkably occurs. However, in oneembodiment of the present invention, even when the output-sidetransistors 101 and 102 in each of the sequential circuits 10_1 to 10_(—) y are normally on, the transistors 101 and 102 can be turned offwhen they should be turned off.

Thus, the semiconductor device of one embodiment of the presentinvention that includes the above shift register consumes less power andcan prevent a decrease in amplitude of the output signals GOUT 1 to GOUTy. A semiconductor display device of one embodiment of the presentinvention that includes the above shift register consumes less power andcan prevent a display defect due to small amplitude of a signal suppliedto the bus line.

Specific Configuration Example 2 of Sequential Circuit

Another configuration examples of the sequential circuit of oneembodiment of the present invention are described.

The sequential circuit 10 illustrated in FIG. 7A includes the circuit11, the transistor 101, and the transistor 102. The transistors 101 and102 correspond to the transistors 12 and 13, respectively, in FIG. 1A.In the sequential circuit 10, a variety of power supply potentials aresupplied through the wirings 110, 111, and 112, and the clock signalsCLKA1 and CLKA2 are supplied through the wirings 113 and 114,respectively. The clock signal CLKB is supplied through the wiring 116,the input signal LIN is supplied through the wiring 117, and the inputsignal RIN is supplied through the wiring 118. In the sequential circuit10, the output signal SROUT is output through the wiring 119, and theoutput signal GOUT is output through the wiring 120.

In the sequential circuit 10 illustrated in FIG. 7A, the circuit 11includes transistors 313 to 319.

A shift register can be constituted by connecting the sequentialcircuits 10 in the plurality of stages to each other.

A gate of the transistor 313 is connected to gates of the transistors314 and 101. One of a source and a drain of the transistor 313 isconnected to the wiring 111. The other of the source and the drain ofthe transistor 313 is connected to gates of the transistors 319 and 102.One of a source and a drain of the transistor 314 is connected to thewiring 111, and the other of the source and the drain of the transistor314 is connected to the wiring 119. One of a source and a drain of thetransistor 101 is connected to the wiring 112, and the other of thesource and the drain of the transistor 101 is connected to the wiring120.

A gate of the transistor 315 is connected to the wiring 117. One of asource and a drain of the transistor 315 is connected to the wiring 110.The other of the source and the drain of the transistor 315 is connectedto the gates of the transistors 319 and 102. A gate of the transistor316 is connected to the wiring 114. One of a source and a drain of thetransistor 316 is connected to the wiring 110. The other of the sourceand the drain of the transistor 316 is connected to the gates of thetransistors 313, 314, and 101. A gate of the transistor 317 is connectedto the wiring 118. One of a source and a drain of the transistor 317 isconnected to the wiring 110. The other of the source and the drain ofthe transistor 317 is connected to the gates of the transistors 313,314, and 101.

A gate of the transistor 318 is connected to the wiring 117. One of asource and a drain of the transistor 318 is connected to the wiring 111.The other of the source and the drain of the transistor 318 is connectedto gates of the transistors 313, 314, and 101. The gate of thetransistor 319 is connected to the gate of the transistor 102. One of asource and a drain of the transistor 319 is connected to the wiring 113.The other of the source and the drain of the transistor 319 is connectedto the wiring 119. The gate of the transistor 102 is connected to thegate of the transistor 319. One of a source and a drain of thetransistor 102 is connected to the wiring 116. The other of the sourceand the drain of the transistor 102 is connected to the wiring 120.

FIG. 7A illustrates an example of the sequential circuit 10 in which allof the transistors are n-channel transistors. Specifically, FIG. 7Aillustrates a case where the power supply potential VDD is supplied tothe wiring 110, the power supply potential VSS is supplied to the wiring111, and the power supply potential VEE is supplied to the wiring 112 asan example.

In the sequential circuit 10 in FIG. 7A, the gate and the one of thesource and the drain of the output-side transistor 101 can beelectrically isolated from each other. Thus, even when the transistor101 is normally on and thus the potential of the wiring 112 forsupplying a potential to the one of the source and the drain of thetransistor 101 is raised, the transistor 101 can be turned off when itshould be turned off. Since the potential VEE of the clock signal CLKBthat is higher than the power supply potential VSS is supplied to theone of the source and the drain of the transistor 102, the gate voltageof the transistor 102 can be lower than the threshold voltage Vth. Thus,even when the transistor 102 is normally on, the transistor 102 can beturned off when it should be turned off.

The sequential circuit 10 in FIG. 7B includes the circuit 11, thetransistor 101, and the transistor 102. The transistors 101 and 102correspond to the transistors 12 and 13, respectively, in FIG. 1A. Inthe sequential circuit 10, a variety of power supply potentials aresupplied through the wirings 110, 111, and 112, and the clock signalsCLKA1, CLKA2, and CLKA3 are supplied through wirings 113, 114, and 115,respectively. The clock signal CLKB is supplied through the wiring 116,the input signal LIN is supplied through the wiring 117, and the inputsignal RIN is supplied through the wiring 118. In the sequential circuit10, the output signal SROUT is output through the wiring 119, and theoutput signal GOUT is output through the wiring 120.

In the sequential circuit 10 in FIG. 7B, the circuit 11 includestransistors 344 to 351.

A shift register can be constituted by connecting the sequentialcircuits 10 in the plurality of stages to each other.

A gate of the transistor 344 is connected to gates of the transistors345 and 101. One of a source and a drain of the transistor 344 isconnected to the wiring 111. The other of the source and the drain ofthe transistor 344 is connected gates of the transistors 351 and 102.One of a source and a drain of the transistor 345 is connected to thewiring 111, and the other of the source and the drain of the transistor345 is connected to the wiring 119. One of a source and a drain of thetransistor 101 is connected to the wiring 112, and the other of thesource and the drain of the transistor 101 is connected to the wiring120.

A gate of the transistor 346 is connected to the wiring 117. One of asource and a drain of the transistor 346 is connected to the wiring 110.The other of the source and the drain of the transistor 346 is connectedto the gates of the transistors 351 and 102. A gate of the transistor347 is connected to the wiring 114. One of a source and a drain of thetransistor 347 is connected to the wiring 110. The other of the sourceand the drain of the transistor 347 is connected to the gates of thetransistors 344, 345, and 101. A gate of the transistor 348 is connectedto the wiring 115. One of a source and a drain of the transistor 348 isconnected to the wiring 110. The other of the source and the drain ofthe transistor 348 is connected to the gates of the transistors 344,345, and 101. A gate of the transistor 349 is connected to the wiring117. One of a source and a drain of the transistor 349 is connected tothe wiring 111. The other of the source and the drain of the transistor349 is connected to the gates of the transistors 344, 345, and 101.

A gate of the transistor 350 is connected to the wiring 118. One of asource and a drain of the transistor 350 is connected to the wiring 110.The other of the source and the drain of the transistor 350 is connectedto gates of the transistors 344, 345, and 101. The gate of thetransistor 351 is connected to the gate of the transistor 102. One of asource and a drain of the transistor 351 is connected to the wiring 113.The other of the source and the drain of the transistor 351 is connectedto the wiring 119. The gate of the transistor 102 is connected to thegate of the transistor 351. One of a source and a drain of thetransistor 102 is connected to the wiring 116, and the other of thesource and the drain of the transistor 102 is connected to the wiring120.

FIG. 7B illustrates an example of the sequential circuit 10 in which allof the transistors are n-channel transistors. Specifically, FIG. 7Billustrates a case where the power supply potential VDD is supplied tothe wiring 110, the power supply potential VSS is supplied to the wiring111, and the power supply potential VEE is supplied to the wiring 112 asan example.

In the sequential circuit 10 in FIG. 7B, the gate and the one of thesource and the drain of the output-side transistor 101 can beelectrically isolated from each other. Thus, even when the transistor101 is normally on and thus the potential of the wiring 112 forsupplying a potential to the one of the source and the drain of thetransistor 101 is raised, the transistor 101 can be turned off when itshould be turned off. Since the potential VEE of the clock signal CLKBthat is higher than the power supply potential VSS is supplied to theone of the source and the drain of the transistor 102, the gate voltageof the transistor 102 can be lower than the threshold voltage Vth. Thus,even when the transistor 102 is normally on, the transistor 102 can beturned off when it should be turned off.

The sequential circuit 10 in FIG. 8A includes the circuit 11, thetransistor 101, and the transistor 102. The transistors 101 and 102correspond to the transistors 12 and 13, respectively, in FIG. 1A. Inthe sequential circuit 10, a variety of power supply potentials aresupplied through the wirings 110, 111, and 112, and the clock signalsCLKA1 and CLKA2 are supplied through the wirings 113 and 114,respectively. The clock signal CLKB is supplied through the wiring 116,the input signal LIN is supplied through the wiring 117, and the inputsignal RIN is supplied through the wiring 118. In the sequential circuit10, the output signal SROUT is output through the wiring 119, and theoutput signal GOUT is output through the wiring 120.

In the sequential circuit 10 in FIG. 8A, the circuit 11 includestransistors 374 to 381.

A shift register can be constituted by connecting the sequentialcircuits 10 in the plurality of stages to each other.

A gate of the transistor 374 is connected to gates of the transistors375 and 101. One of a source and a drain of the transistor 374 isconnected to the wiring 111. The other of the source and the drain ofthe transistor 374 is connected to one of a source and a drain of thetransistor 377. One of a source and a drain of the transistor 375 isconnected to the wiring 111, and the other of the source and the drainof the transistor 375 is connected to the wiring 119. One of a sourceand a drain of the transistor 101 is connected to the wiring 112, andthe other of the source and the drain of the transistor 101 is connectedto the wiring 120.

A gate of the transistor 376 is connected to the wiring 117. One of asource and a drain of the transistor 376 is connected to the wiring 110.The other of the source and the drain of the transistor 376 is connectedto the one of the source and the drain of the transistor 377. A gate ofthe transistor 377 is connected to the wiring 110. The other of thesource and the drain of the transistor 377 is connected to gates of thetransistors 381 and 102. A gate of the transistor 378 is connected tothe wiring 114. One of a source and a drain of the transistor 378 isconnected to the wiring 110. The other of the source and the drain ofthe transistor 378 is connected to the gates of the transistors 374,375, and 101.

A gate of the transistor 379 is connected to the wiring 117. One of asource and a drain of the transistor 379 is connected to the wiring 111.The other of the source and the drain of the transistor 379 is connectedto the gates of the transistors 374, 375, and 101. A gate of thetransistor 380 is connected to the wiring 118. One of a source and adrain of the transistor 380 is connected to the wiring 110. The other ofthe source and the drain of the transistor 380 is connected to the gatesof the transistors 374, 375, and 101. One of a source and a drain of thetransistor 381 is connected to the wiring 113. The other of the sourceand the drain of the transistor 381 is connected to the wiring 119. Oneof a source and a drain of the transistor 102 is connected to the wiring116. The other of the source and the drain of the transistor 102 isconnected to the wiring 120.

FIG. 8A illustrates an example of the sequential circuit 10 in which allof the transistors are n-channel transistors. Specifically, FIG. 8Aillustrates a case where the power supply potential VDD is supplied tothe wiring 110, the power supply potential VSS is supplied to the wiring111, and the power supply potential VEE is supplied to the wiring 112 asan example.

In the sequential circuit 10 in FIG. 8A, the gate and the one of thesource and the drain of the output-side transistor 101 can beelectrically isolated from each other. Thus, even when the transistor101 is normally on and thus the potential of the wiring 112 forsupplying a potential to the one of the source and the drain of thetransistor 101 is raised, the transistor 101 can be turned off when itshould be turned off. Since the potential VEE of the clock signal CLKBthat is higher than the power supply potential VSS is supplied to theone of the source and the drain of the transistor 102, the gate voltageof the transistor 102 can be lower than the threshold voltage Vth. Thus,even when the transistor 102 is normally on, the transistor 102 can beturned off when it should be turned off.

The sequential circuit 10 in FIG. 8B includes the circuit 11, thetransistor 101, and the transistor 102. The transistors 101 and 102correspond to the transistors 12 and 13, respectively, in FIG. 1A. Inthe sequential circuit 10, a variety of power supply potentials aresupplied through the wirings 110, 111, and 112, and the clock signalsCLKA1 and CLKA2 are supplied through the wirings 113 and 114,respectively. The clock signal CLKB is supplied through the wiring 116,the input signal LIN is supplied through the wiring 117, and the inputsignal RIN is supplied through the wiring 118. In the sequential circuit10, the output signal SROUT is output through the wiring 119, and theoutput signal GOUT is output through the wiring 120.

In the sequential circuit 10 in FIG. 8B, the circuit 11 includestransistors 414 to 422.

A shift register can be constituted by connecting the sequentialcircuits 10 in the plurality of stages to each other.

A gate of the transistor 414 is connected to gates of the transistors415 and 101. One of a source and a drain of the transistor 414 isconnected to the wiring 111. The other of the source and the drain ofthe transistor 414 is connected to one of a source and a drain of thetransistor 417. One of a source and a drain of the transistor 415 isconnected to the wiring 111, and the other of the source and the drainof the transistor 415 is connected to the wiring 119. One of a sourceand a drain of the transistor 101 is connected to the wiring 112, andthe other of the source and the drain of the transistor 101 is connectedto the wiring 120.

A gate of the transistor 416 is connected to the wiring 117. One of asource and a drain of the transistor 416 is connected to the wiring 110.The other of the source and the drain of the transistor 416 is connectedto the one of the source and the drain of the transistor 417. A gate ofthe transistor 417 is connected to the wiring 110. The other of thesource and the drain of the transistor 417 is connected to a gate of thetransistor 421. A gate of the transistor 418 is connected to the wiring114. One of a source and a drain of the transistor 418 is connected tothe wiring 110. The other of the source and the drain of the transistor418 is connected to the gates of the transistors 414, 415, and 101. Agate of the transistor 419 is connected to the wiring 117. One of asource and a drain of the transistor 419 is connected to the wiring 111.The other of the source and the drain of the transistor 419 is connectedto the gates of the transistors 414, 415, and 101. A gate of thetransistor 420 is connected to the wiring 118. One of a source and adrain of the transistor 420 is connected to the wiring 110. The other ofthe source and the drain of the transistor 420 is connected to the gatesof the transistors 414, 415, and 101. One of a source and a drain of thetransistor 421 is connected to the wiring 113. The other of the sourceand the drain of the transistor 421 is connected to the wiring 119. Agate of the transistor 422 is connected to the wiring 110. One of asource and a drain of the transistor 422 is connected to the gate of thetransistor 421 and the other of the source and the drain of thetransistor 417. The other of the source and the drain of the transistor422 is connected to a gate of the transistor 102. One of a source and adrain of the transistor 102 is connected to the wiring 116. The other ofthe source and the drain of the transistor 102 is connected to thewiring 120.

FIG. 8B illustrates an example of the sequential circuit 10 in which allof the transistors are n-channel transistors. Specifically, FIG. 8Billustrates a case where the power supply potential VDD is supplied tothe wiring 110, the power supply potential VSS is supplied to the wiring111, and the power supply potential VEE is supplied to the wiring 112 asan example.

In the sequential circuit 10 in FIG. 8B, the gate and the one of thesource and the drain of the output-side transistor 101 can beelectrically isolated from each other. Thus, even when the transistor101 is normally on and thus the potential of the wiring 112 forsupplying a potential to the one of the source and the drain of thetransistor 101 is raised, the transistor 101 can be turned off when itshould be turned off. Since the potential VEE of the clock signal CLKBthat is higher than the power supply potential VSS is supplied to theone of the source and the drain of the transistor 102, the gate voltageof the transistor 102 can be lower than the threshold voltage Vth. Thus,even when the transistor 102 is normally on, the transistor 102 can beturned off when it should be turned off.

The sequential circuit 10 in FIG. 9 includes the circuit 11, thetransistor 101, and the transistor 102. The transistors 101 and 102correspond to the transistors 12 and 13, respectively, in FIG. 1A. Inthe sequential circuit 10, a variety of power supply potentials aresupplied through the wirings 110, 111, and 112, and the clock signalsCLKA1 and CLKA2 are supplied through the wirings 113 and 114,respectively. The clock signal CLKB is supplied through the wiring 116,the input signal LIN is supplied through the wiring 117, and the inputsignal RIN is supplied through the wiring 118. In the sequential circuit10, the output signal SROUT is output through the wiring 119, and theoutput signal GOUT is output through the wiring 120.

In the sequential circuit 10 in FIG. 9, the circuit 11 includestransistors 444 to 452.

A shift register can be constituted by connecting the sequentialcircuits 10 in the plurality of stages to each other.

A gate of the transistor 444 is connected to gates of the transistors445 and 101. One of a source and a drain of the transistor 444 isconnected to the wiring 111. The other of the source and the drain ofthe transistor 444 is connected to one of a source and a drain of thetransistor 452. One of a source and a drain of the transistor 445 isconnected to the wiring 111, and the other of the source and the drainof the transistor 445 is connected to the wiring 119. One of a sourceand a drain of the transistor 101 is connected to the wiring 112, andthe other of the source and the drain of the transistor 101 is connectedto the wiring 120.

A gate of the transistor 446 is connected to the wiring 117. One of asource and a drain of the transistor 446 is connected to the wiring 110.The other of the source and the drain of the transistor 446 is connectedto the one of the source and the drain of the transistor 452. A gate ofthe transistor 447 is connected to the wiring 114. One of a source and adrain of the transistor 447 is connected to the wiring 110. The other ofthe source and the drain of the transistor 447 is connected to the gatesof the transistors 444, 445, and 101. A gate of the transistor 448 isconnected to the wiring 118. One of a source and a drain of thetransistor 448 is connected to the wiring 110. The other of the sourceand the drain of the transistor 448 is connected to the gates of thetransistors 444, 445, and 101. A gate of the transistor 449 is connectedto the wiring 117. One of a source and a drain of the transistor 449 isconnected to the wiring 111. The other of the source and the drain ofthe transistor 449 is connected to the gates of the transistors 444,445, and 101. A gate of the transistor 450 is connected to the wiring110. One of a source and a drain of the transistor 450 is connected tothe one of the source and the drain of the transistor 452. The other ofthe source and the drain of the transistor 450 is connected to the gateof the transistor 451. One of a source and a drain of the transistor 451is connected to the wiring 113. The other of the source and the drain ofthe transistor 451 is connected to the wiring 119. A gate of thetransistor 452 is connected to the wiring 110. The other of the sourceand the drain of the transistor 452 is connected to a gate of thetransistor 102. One of a source and a drain of the transistor 102 isconnected to the wiring 116. The other of the source and the drain ofthe transistor 102 is connected to the wiring 120.

FIG. 9 illustrates an example of the sequential circuit 10 in which allof the transistors are n-channel transistors. Specifically, FIG. 9illustrates a case where the power supply potential VDD is supplied tothe wiring 110, the power supply potential VSS is supplied to the wiring111, and the power supply potential VEE is supplied to the wiring 112 asan example.

In the sequential circuit 10 in FIG. 9, the gate and the one of thesource and the drain of the output-side transistor 101 can beelectrically isolated from each other. Thus, even when the transistor101 is normally on and thus the potential of the wiring 112 forsupplying a potential to the one of the source and the drain of thetransistor 101 is raised, the transistor 101 can be turned off when itshould be turned off. Since the potential VEE of the clock signal CLKBthat is higher than the power supply potential VSS is supplied to theone of the source and the drain of the transistor 102, the gate voltageof the transistor 102 can be lower than the threshold voltage Vth. Thus,even when the transistor 102 is normally on, the transistor 102 can beturned off when it should be turned off.

Structure Example of Semiconductor Display Device

Next, a structure example of a semiconductor display device of oneembodiment of the present invention is described.

In a semiconductor display device 70 illustrated in FIG. 10A, a pixelportion 71 includes a plurality of pixels 55, wirings GL (wirings GL1 toGLy, y: a natural number) that correspond to bus lines each selectingthe pixels 55 in a row, and wirings SL (wirings SL1 to SLx, x: a naturalnumber) for supplying video signals to the selected pixels 55. The inputof signals to the wirings GL is controlled by a driver circuit 72. Theinput of video signals to the wirings SL is controlled by a drivercircuit 73. Each of the plurality of pixels 55 is connected to at leastone of the wirings GL and at least one of the wirings SL.

Specifically, the driver circuit 72 includes a shift register 75 thatproduces signals for sequentially selecting wirings GL1 to GLy.Moreover, specifically, the driver circuit 73 includes a shift register76 that sequentially produces signals having pulses and a switchingcircuit 77 that controls supply of video signals to wirings SL1 to SLxin accordance with the signals produced in the shift register 76.

The sequential circuit of one embodiment of the present invention can beused for one or both of the shift registers 75 and 76.

Note that the kinds and number of the wirings in the pixel portion 71can be determined by the structure, number, and position of the pixels55. Specifically, in the pixel portion 71 illustrated in FIG. 10A, thepixels 55 are arranged in a matrix of x columns and y rows, and thewirings SL1 to SLx and the wirings GL1 to GLy are provided in the pixelportion 71 as an example.

Although FIG. 10A illustrates the case where the driver circuits 72 and73 and the pixel portion 71 are formed over one substrate as an example,the driver circuits 72 and 73 may be formed over a substrate differentfrom a substrate over which the pixel portion 71 is formed.

FIG. 10B illustrates an example of a configuration of the pixel 55. Eachof the pixels 55 includes a liquid crystal element 60, a transistor 56that controls the supply of an video signal to the liquid crystalelement 60, and a capacitor 57 that holds voltage between a pixelelectrode and a common electrode of the liquid crystal element 60. Theliquid crystal element 60 includes the pixel electrode, the commonelectrode, and a liquid crystal layer containing a liquid crystalmaterial to which voltage between the pixel electrode and the commonelectrode is applied.

The transistor 56 controls whether to supply the potential of the wiringSL to the pixel electrode of the liquid crystal element 60. Apredetermined potential is supplied to the common electrode of theliquid crystal element 60.

The connection state between the transistor 56 and the liquid crystalelement 60 is specifically described below. In FIG. 10B, a gate of thetransistor 56 is connected to any one of the wirings GL1 to GLy. One ofa source and a drain of the transistor 56 is connected to any one of thewirings SL1 to SLx, and the other is connected to the pixel electrode ofthe liquid crystal element 60.

The transmittance of the liquid crystal element 60 changes when thealignment of liquid crystal molecules included in the liquid crystallayer changes in accordance with the level of voltage applied betweenthe pixel electrode and the common electrode. Accordingly, when thetransmittance of the liquid crystal element 60 is controlled by thepotential of an video signal supplied to the pixel electrode, gray-scaleimages can be displayed. In each of the plurality of pixels 55 includedin the pixel portion 71, the gray level of the liquid crystal element 60is adjusted in response to an video signal containing image data; thus,an image is displayed on the pixel portion 71.

FIG. 10B illustrates an example in which the one transistor 56 is usedas a switch for controlling the input of an video signal to the pixel55. However, a plurality of transistors functioning as one switch may beused in the pixel 55.

In one embodiment of the present invention, the transistor 56 withextremely low off-state current is preferably used as the switch forcontrolling the input of an video signal to the pixel 55. With thetransistor 56 having extremely low off-state current, leakage of chargethrough the transistor 56 can be prevented. Thus, the potential of anvideo signal that is supplied to the liquid crystal element 60 and thecapacitor 57 can be held more reliably. Accordingly, changes intransmittance of the liquid crystal element 60 due to leakage of chargein one frame period are prevented, so that the quality of an image to bedisplayed can be improved. Since leakage of charge through thetransistor 56 can be prevented when the transistor 56 has low off-statecurrent, the supply of a power supply potential or a signal to thedriver circuits 72 and 73 may be stopped in a period during which astill image is displayed. With the above configuration, the number oftimes of writing video signals to the pixel portion 71 can be reduced,and thus power consumption of the semiconductor display device can bereduced.

For example, the off-state current of a transistor including asemiconductor film containing an oxide semiconductor can be extremelylow, and therefore is suitable for the transistor 56, for example.

In addition, the transistor 56 in FIG. 10B may include a pair of gateelectrodes overlapping with each other with a semiconductor filmprovided therebetween. The pair of gate electrodes are electricallyconnected to each other. In one embodiment of the present invention, theabove structure allows the on-state current and the reliability of thetransistor 56 to be increased.

Next, FIG. 10C illustrates another example of the pixel 55. The pixel 55includes a transistor 95 for controlling input of a video signal to thepixel 55, a light-emitting element 98, a transistor 96 for controllingthe value of current supplied to the light-emitting element 98 inresponse to an video signal, and a capacitor 97 for holding thepotential of an video signal.

Examples of the light-emitting element 98 include an element whoseluminance is controlled by current or voltage, such as a light-emittingdiode (LED) or an organic light-emitting diode (OLED). For example, anOLED includes at least an EL layer, an anode, and a cathode. The ELlayer is formed using a single layer or a plurality of layers betweenthe anode and the cathode, at least one of which is a light-emittinglayer containing a light-emitting substance.

From the EL layer, electroluminescence is obtained by current suppliedwhen a potential difference between the cathode and the anode is higherthan or equal to the threshold voltage of the light-emitting element 98.As electroluminescence, there are luminescence (fluorescence) at thetime of returning from a singlet-excited state to a ground state andluminescence (phosphorescence) at the time of returning from atriplet-excited state to a ground state.

The potential of one of the anode and the cathode of the light-emittingelement 98 is controlled in response to an video signal input to thepixel 55. The one of the anode and the cathode whose potential iscontrolled in response to a video signal is used as a pixel electrode,and the other is used as a common electrode. A predetermined potentialis supplied to the common electrode of the light-emitting element 98,and the luminance of the light-emitting element 98 is determined by apotential difference between the pixel electrode and the commonelectrode. Thus, the luminance of the light-emitting element 98 iscontrolled by the potential of the video signal, so that thelight-emitting element 98 can express gray level. In each of theplurality of pixels 55 included in the pixel portion, the gray level ofthe light-emitting element 98 is adjusted in response to an video signalcontaining image data; thus, an image is displayed on the pixel portion71.

Next, connection between the transistor 95, the transistor 96, thecapacitor 97, and the light-emitting element 98 that are included in thepixel 55 is described.

One of a source and a drain of the transistor 95 is connected to thewiring SL, and the other is connected to a gate of the transistor 96. Agate of the transistor 95 is connected to the wiring GL. One of a sourceand a drain of the transistor 96 is connected to a power supply line VL,and the other is connected to the light-emitting element 98.Specifically, the other of the source and the drain of the transistor 96is connected to one of the anode and the cathode of the light-emittingelement 98. A predetermined potential is supplied to the other of theanode and the cathode of the light-emitting element 98.

FIG. 10C illustrates the case where the transistor 96 includes a pair ofgate electrodes overlapping with each other with a semiconductor filmprovided therebetween. The pair of gate electrodes are electricallyconnected to each other. In one embodiment of the present invention, theabove structure allows the on-state current and the reliability of thetransistor 96 to be increased.

<Structure of Pixel>

Next, description is given of a structure example of the pixel 55 in aliquid crystal display device that is an example of the semiconductordisplay device 70 illustrated in FIG. 10A. FIG. 11 illustrates as anexample of a top view of the pixel 55. Insulating films are notillustrated in FIG. 11 in order to clarify the layout of the pixel 55.FIG. 12 is a cross-sectional view of the liquid crystal display deviceusing an element substrate including the pixel 55 illustrated in FIG.11. In the liquid crystal display device in FIG. 12, the elementsubstrate including a substrate 31 corresponds to a cross-sectional viewalong the dashed line B1-B2 in FIG. 11.

The pixel 55 illustrated in FIG. 11 and FIG. 12 includes the transistor56 and the capacitor 57. In FIG. 12, the pixel 55 includes the liquidcrystal element 60.

Over the substrate 31 having an insulating surface, the transistor 56includes a conductive film 40 serving as a gate electrode, an insulatingfilm 22 that is over the conductive film 40 and serves as a gateinsulating film, an oxide semiconductor film 41 that is over theinsulating film 22 and overlaps with the conductive film 40, and aconductive film 43 and a conductive film 44 that are electricallyconnected to the oxide semiconductor film 41 and serve as a sourceelectrode and a drain electrode. The conductive film 40 serves as thewiring GL illustrated in FIG. 10B. The conductive film 43 serves as thewiring SL illustrated in FIG. 10B.

The pixel 55 includes a metal oxide film 42 over the insulating film 22.The metal oxide film 42 is a conductive film that transmits visiblelight. A conductive film 61 electrically connected to the metal oxidefilm 42 is provided over the metal oxide film 42. The conductive film 61serves as a wiring that supplies a predetermined potential to the metaloxide film 42.

The insulating film 22 may be formed using a single layer or a stackedlayer of an insulating film containing one or more kinds of aluminumoxide, magnesium oxide, silicon oxide, silicon oxynitride, siliconnitride oxide, silicon nitride, gallium oxide, germanium oxide, yttriumoxide, zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide,and tantalum oxide. Note that in this specification, oxynitride containsmore oxygen than nitrogen, and nitride oxide contains more nitrogen thanoxygen.

In FIG. 12, an insulating film 26 and an insulating film 27 are stackedin this order provided over the oxide semiconductor film 41, theconductive film 43, the conductive film 44, the metal oxide film 42, andthe conductive film 61. The transistor 56 may include the insulatingfilms 26 and 27. Although the insulating films 26 and 27, which arestacked in this order, are illustrated in FIG. 12, a single insulatingfilm or a stack of three or more insulating films may be used instead ofthe insulating films 26 and 27.

An opening 58 is provided in the insulating films 26 and 27 to overlapwith the metal oxide film 42. The opening 58 is provided in a regionoverlapping with the metal oxide film 42, and the oxide semiconductorfilm 41, the conductive film 43, and the conductive film 44 are notprovided in the region.

In FIG. 12, a nitride insulating film 28 and an insulating film 29 arestacked in this order over the insulating film 26 and the insulatingfilm 27 and over the metal oxide film 42 in the opening 58.

Note that by forming an oxide semiconductor film over the insulatingfilm 22 and forming the nitride insulating film 28 to be in contact withthe oxide semiconductor film, the conductivity of the oxidesemiconductor film can be increased. In that case, the oxidesemiconductor film with high conductivity can be used as the metal oxidefilm 42. The conductivity of the oxide semiconductor film is increasedprobably because oxygen vacancies are formed in the oxide semiconductorfilm at the time of forming the opening 58 or the nitride insulatingfilm 28, and hydrogen diffused from the nitride insulating film 28 isbonded to the oxygen vacancies to form a donor. Specifically, theresistivity of the metal oxide film 42 is higher than or equal to 1×10⁻³Ωcm and lower than 1×10⁴ Ωcm, preferably higher than or equal to 1×10⁻³Ωcm and lower than 1×10⁻¹ Ωcm.

It is preferable that the metal oxide film 42 have a higher hydrogenconcentration than the oxide semiconductor film 41. In the metal oxidefilm 42, the hydrogen concentration measured by secondary ion massspectrometry (SIMS) is greater than or equal to 8×10¹⁹ atoms/cm³,preferably greater than or equal to 1×10²⁰ atoms/cm³, more preferablygreater than or equal to 5×10²⁰ atoms/cm³. In the oxide semiconductorfilm 41, the hydrogen concentration measured by SIMS is less than 5×10¹⁹atoms/cm³, preferably less than 5×10¹⁸ atoms/cm³, further preferablyless than or equal to 1×10¹⁸ atoms/cm³, still further preferably lessthan or equal to 5×10¹⁷ atoms/cm³, yet still further preferably lessthan or equal to 1×10¹⁶ atoms/cm³.

For the nitride insulating film 28, silicon nitride, silicon nitrideoxide, aluminum nitride, or aluminum nitride oxide can be used, forexample. In comparison with an oxide insulating film such as a siliconoxide film and an aluminum oxide film, the nitride insulating film 28containing any of the above materials can further prevent impuritiesfrom outside, such as water, alkali metal, and alkaline-earth metal,from being diffused into the oxide semiconductor film 41.

Furthermore, an opening 62 is provided in the nitride insulating film 28and the insulating film 29 to overlap with the conductive film 44. Aconductive film 45 that transmits visible light and serves as a pixelelectrode is provided over the nitride insulating film 28 and theinsulating film 29. The conductive film 45 is electrically connected tothe conductive film 44 in the opening 62. The conductive film 45overlaps with the metal oxide film 42 in the opening 58. A portion wherethe conductive film 45 and the metal oxide film 42 overlap with eachother with the nitride insulating film 28 and the insulating film 29sandwiched therebetween serves as the capacitor 57.

In the capacitor 57, the metal oxide film 42 and the conductive film 45serving as a pair of electrodes and the nitride insulating film 28 andthe insulating film 29 collectively serving as a dielectric filmtransmit visible light. This means that the capacitor 57 transmitsvisible light. Thus, the aperture ratio of the pixel 55 can be higherthan that of a pixel including a capacitor having a property oftransmitting less visible light. Therefore, the required capacitance forhigh image quality can be secured and the aperture ratio of the pixelcan be increased; thus, light loss can be reduced in a panel and powerconsumption of a semiconductor device can be reduced.

Note that as described above, the insulating film 29 is not necessarilyprovided. However, with the use of the insulating film 29 using aninsulator, which has a dielectric constant lower than that of thenitride insulating film 28, as a dielectric film together with thenitride insulating film 28, the dielectric constant of the dielectricfilm of the capacitor 57 can be adjusted to a desired value withoutincreasing the thickness of the nitride insulating film 28.

An alignment film 52 is provided over the conductive film 45.

A substrate 46 is provided to face the substrate 31. A shielding film 47blocking visible light and a coloring layer 48 transmitting visiblelight in a specific wavelength range are provided on the substrate 46. Aresin film 50 is provided on the shielding film 47 and the coloringlayer 48, and a conductive film 59 serving as a common electrode isprovided on the resin film 50. An alignment film 51 is provided on theconductive film 59.

Between the substrates 31 and 46, a liquid crystal layer 53 containing aliquid crystal material is sandwiched between the alignment films 52 and51. The liquid crystal element 60 includes the conductive film 45, theconductive film 59, and the liquid crystal layer 53.

Although a twisted nematic (TN) mode is used as a method for driving theliquid crystal in FIG. 11 and FIG. 12, the following can be used as amethod for driving the liquid crystal: a fringe field switching (FFS)mode, a super twisted nematic (STN) mode, a vertical alignment (VA)mode, a multi-domain vertical alignment (MVA) mode, anin-plane-switching (IPS) mode, an optically compensated birefringence(OCB) mode, a blue phase mode, a transverse bend alignment (TBA) mode, aVA-IPS mode, an electrically controlled birefringence (ECB) mode, aferroelectric liquid crystal (FLC) mode, an anti-ferroelectric liquidcrystal (AFLC) mode, a polymer dispersed liquid crystal (PDLC) mode, apolymer network liquid crystal (PNLC) mode, a guest-host mode, anadvanced super view (ASV) mode, and the like.

In the liquid crystal display device of one embodiment of the presentinvention, the liquid crystal layer can be formed using, for example, aliquid crystal material classified into a thermotropic liquid crystal ora lyotropic liquid crystal. As another example of a liquid crystalmaterial used for the liquid crystal layer, the following can be given:a nematic liquid crystal, a smectic liquid crystal, a cholesteric liquidcrystal, or a discotic liquid crystal. Further alternatively, a liquidcrystal material categorized by a ferroelectric liquid crystal or ananti-ferroelectric liquid crystal can be used. Further alternatively, aliquid crystal material categorized by a high-molecular liquid crystalsuch as a main-chain high-molecular liquid crystal, a side-chainhigh-molecular liquid crystal, or a composite-type high-molecular liquidcrystal, or a low-molecular liquid crystal can be used. Furtheralternatively, a liquid crystal material categorized by a polymerdispersed liquid crystal (PDLC) can be used.

Alternatively, liquid crystal exhibiting a blue phase for which analignment film is unnecessary may be used for the liquid crystal layer.A blue phase is one of liquid crystal phases, which is generated justbefore a cholesteric phase changes into an isotropic phase whiletemperature of cholesteric liquid crystal is increased. Since the bluephase is only generated within a narrow range of temperature, a chiralmaterial or an ultraviolet curable resin is added so that thetemperature range is improved. The liquid crystal composition thatincludes a liquid crystal exhibiting a blue phase and a chiral materialis preferable because it has a small response time of less than or equalto 1 msec, has optical isotropy, which makes the alignment processunneeded, and has a small viewing angle dependence.

Although a liquid crystal display device using a color filter to displaya color image is illustrated in FIG. 12 as an example, the liquidcrystal display device of one embodiment of the present invention maydisplay a color image by sequentially lighting a plurality of lightsources having different hues.

The oxide semiconductor film 41 of the transistor 56 is not necessarilya single oxide semiconductor film, but may be a stack of a plurality ofoxide semiconductor films. FIG. 13A illustrates an example in which theoxide semiconductor film 41 is formed using a stack of three oxidesemiconductor films. Specifically, in the transistor 56 in FIG. 13A,oxide semiconductor films 41 a, 41 b, and 41 c are stacked sequentiallyfrom the insulating film 22 side as the oxide semiconductor film 41.

The oxide semiconductor films 41 a and 41 c each contains at least oneof metal elements contained in the oxide semiconductor film 41 b. Theenergy at the bottom of the conduction band of the oxide semiconductorfilms 41 a and 41 c is closer to a vacuum level than that of the oxidesemiconductor film 41 b by 0.05 eV or more, 0.07 eV or more, 0.1 eV ormore, or 0.15 eV or more and 2 eV or less, 1 eV or less, 0.5 eV or less,or 0.4 eV or less. Further, the oxide semiconductor film 41 b preferablycontains at least indium in order that the carrier mobility is high.

As illustrated in FIG. 13B, the oxide semiconductor film 41 coverlapping with the insulating film 22 may be provided over theconductive films 43 and 44.

<Top and Cross-Sectional Views of Semiconductor Display Device>

The appearance of a semiconductor display device of one embodiment ofthe present invention is described with reference to FIG. 14. FIG. 14 isa top view of a liquid crystal display device where a substrate 4001 anda substrate 4006 are bonded to each other with a sealant 4005. FIG. 15corresponds to a cross-sectional view taken along dashed line C1-C2 inFIG. 14.

The sealant 4005 is provided to surround a pixel portion 4002 and a pairof driver circuits 4004 provided over the substrate 4001. The substrate4006 is provided over the pixel portion 4002 and the driver circuits4004. Thus, the pixel portion 4002 and the driver circuits 4004 aresealed by the substrate 4001, the sealant 4005, and the substrate 4006.

A driver circuit 4003 is mounted in a region that is different from theregion surrounded by the sealant 4005 over the substrate 4001.

A plurality of transistors are included in the pixel portion 4002 andthe driver circuits 4004 provided over the substrate 4001. FIG. 15illustrates a transistor 4010 included in the pixel portion 4002. Aninsulating film 4020 that can be formed using a variety of insulatingfilms including an oxide insulating film and an insulating film 4022that can be formed using a variety of insulating films including anitride insulating film are provided over the transistor 4010. Thetransistor 4010 is connected to a pixel electrode 4021 over theinsulating film 4022 in an opening portion provided in the insulatingfilms 4020 and 4022.

A resin film 4059 is provided on the substrate 4006, and a commonelectrode 4060 is provided on the resin film 4059. A liquid crystallayer 4028 between the pixel electrode 4021 and the common electrode4060 is provided between the substrates 4001 and 4006. A liquid crystalelement 4023 includes the pixel electrode 4021, the common electrode4060, and the liquid crystal layer 4028.

The transmittance of the liquid crystal element 4023 changes when thealignment of liquid crystal molecules included in the liquid crystallayer 4028 changes in accordance with the level of a voltage appliedbetween the pixel electrode 4021 and the common electrode 4060.Accordingly, when the transmittance of the liquid crystal element 4023is controlled by the potential of a video signal supplied to the pixelelectrode 4021, gray-scale images can be displayed.

As illustrated in FIG. 15, in one embodiment of the present invention,the insulating films 4020 and 4022 are removed at an end portion of thepanel. A conductive film 4050 is formed in the region where theinsulating films 4020 and 4022 are removed. The conductive film 4050 anda conductive film serving as a source or a drain of the transistor 4010can be formed by etching one conductive film.

A resin film 4062 in which conductive particles 4061 having conductivityare dispersed is provided between the substrate 4001 and the substrate4006. The conductive film 4050 is electrically connected to the commonelectrode 4060 through the conductive particles 4061. In other words,the common electrode 4060 and the conductive film 4050 are electricallyconnected to each other through the conductive particle 4061 at the endportion of the panel. The resin film 4062 can be formed using athermosetting resin or an ultraviolet curable resin. As the conductiveparticle 4061, a particle of a spherical organic resin coated withthin-film metal of Au, Ni, Co, or the like can be used, for example.

An alignment film is not illustrated in FIG. 15. In the case ofproviding alignment films on the pixel electrode 4021 and the commonelectrode 4060, the alignment film on the common electrode 4060 ispartly removed and the alignment film on the conductive film 4050 ispartly removed; thus, electrical connection can be obtained among thecommon electrode 4060, the conductive particle 4061, and the conductivefilm 4050.

Note that in the liquid crystal display device of one embodiment of thepresent invention, a color image may be displayed by using a colorfilter or by sequentially turning on a plurality of light sourcesemitting light with different hues.

Video signals from the driver circuit 4003 and a variety of controlsignals and potentials from an FPC 4018 are supplied to the drivercircuits 4004 or the pixel portion 4002 through lead wirings 4030 and4031.

<Semiconductor Film>

There are few carrier generation sources in a highly purified oxidesemiconductor (purified oxide semiconductor) obtained by reduction ofimpurities such as moisture and hydrogen serving as electron donors(donors) and reduction of oxygen vacancies; therefore, the highlypurified oxide semiconductor can be an intrinsic (i-type) semiconductoror a substantially i-type semiconductor. For this reason, a transistorhaving a channel formation region in a highly purified oxidesemiconductor film has extremely low off-state current and highreliability. Thus, a transistor in which a channel formation region isformed in the oxide semiconductor film easily has an electricalcharacteristic of a positive threshold voltage (also referred to as anormally-off characteristic).

Specifically, various experiments can prove low off-state current of atransistor having a channel formation region in a highly purified oxidesemiconductor film. For example, even when an element has a channelwidth of 1×10⁶ μm and a channel length of 10 μm, off-state current canbe less than or equal to the measurement limit of a semiconductorparameter analyzer, i.e., less than or equal to 1×10⁻¹³ A, at a voltage(drain voltage) between the source electrode and the drain electrode of1 V to 10 V. In that case, it can be seen that off-state currentstandardized on the channel width of the transistor is lower than orequal to 100 zA/μm. In addition, a capacitor and a transistor areconnected to each other and the off-state current is measured with acircuit in which charge flowing into or from the capacitor is controlledby the transistor. In the measurement, a highly purified oxidesemiconductor film was used for a channel formation region of thetransistor, and the off-state current of the transistor was measuredfrom a change in the amount of charge of the capacitor per unit hour. Asa result, it was found that, in the case where the voltage between thesource electrode and the drain electrode of the transistor is 3 V, alower off-state current of several tens of yA/μm is obtained.Accordingly, the off-state current of the transistor in which the highlypurified oxide semiconductor film is used as a channel formation regionis considerably lower than that of a transistor in which silicon havingcrystallinity is used.

In the case where an oxide semiconductor film is used as a semiconductorfilm, an oxide semiconductor preferably contains at least indium (In) orzinc (Zn). As a stabilizer for reducing variations in electricalcharacteristics of a transistor including the oxide semiconductor, theoxide semiconductor preferably contains gallium (Ga) in addition to Inand Zn. Tin (Sn) is preferably contained as a stabilizer. Hafnium (Hf)is preferably contained as a stabilizer. Aluminum (Al) is preferablycontained as a stabilizer. Zirconium (Zr) is preferably contained as astabilizer.

Among the oxide semiconductors, unlike silicon carbide, gallium nitride,or gallium oxide, an In—Ga—Zn-based oxide, an In—Sn—Zn-based oxide, orthe like has an advantage of high mass productivity because a transistorwith favorable electrical characteristics can be formed by a sputteringmethod or a wet process. Further, unlike silicon carbide, galliumnitride, or gallium oxide, with the use of the In—Ga—Zn-based oxide, atransistor with favorable electrical characteristics can be formed overa glass substrate. Furthermore, a larger substrate can be used.

As another stabilizer, one or more kinds of lanthanoid such as lanthanum(La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm),europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium(Ho), erbium (Er), thulium (Tm), ytterbium (Yb), or lutetium (Lu) may becontained.

As the oxide semiconductor, any of the following oxides can be used, forexample: indium oxide, gallium oxide, tin oxide, zinc oxide, anIn—Zn-based oxide, a Sn—Zn-based oxide, an Al—Zn-based oxide, aZn—Mg-based oxide, a Sn—Mg-based oxide, an In—Mg-based oxide, anIn—Ga-based oxide, an In—Ga—Zn-based oxide (also referred to as IGZO),an In—Al—Zn-based oxide, an In—Sn—Zn-based oxide, a Sn—Ga—Zn-basedoxide, an Al—Ga—Zn-based oxide, a Sn—Al—Zn-based oxide, anIn—Hf—Zn-based oxide, an In—La—Zn-based oxide, an In—Pr—Zn-based oxide,an In—Nd—Zn-based oxide, an In—Ce—Zn-based oxide, an In—Sm—Zn-basedoxide, an In—Eu—Zn-based oxide, an In—Gd—Zn-based oxide, anIn—Tb—Zn-based oxide, an In—Dy—Zn-based oxide, an In—Ho—Zn-based oxide,an In—Er—Zn-based oxide, an In—Tm—Zn-based oxide, an In—Yb—Zn-basedoxide, an In—Lu—Zn-based oxide, an In—Sn—Ga—Zn-based oxide, anIn—Hf—Ga—Zn-based oxide, an In—Al—Ga—Zn-based oxide, anIn—Sn—Al—Zn-based oxide, an In—Sn—Hf—Zn-based oxide, and anIn—Hf—Al—Zn-based oxide.

Note that, for example, an In—Ga—Zn-based oxide means an oxidecontaining In, Ga, and Zn, and there is no limitation on the ratio ofIn, Ga, and Zn. In addition, the In—Ga—Zn-based oxide may contain ametal element other than In, Ga, and Zn. The In—Ga—Zn-based oxide hassufficiently high resistance when no electric field is applied thereto,so that off-state current can be sufficiently reduced. Further, theIn—Ga—Zn-based oxide has high mobility.

For example, with an In—Sn—Zn-based oxide, high mobility can berelatively easily obtained. However, even with an In—Ga—Zn-based oxide,mobility can be increased by lowering defect density in the bulk.

A structure of the oxide semiconductor film is described below.

An oxide semiconductor film is classified roughly into a single-crystaloxide semiconductor film and a non-single-crystal oxide semiconductorfilm. The non-single-crystal oxide semiconductor film includes any of anamorphous oxide semiconductor film, a microcrystalline oxidesemiconductor film, a polycrystalline oxide semiconductor film, aCAAC-OS film, and the like.

The amorphous oxide semiconductor film has disordered atomic arrangementand no crystalline component. A typical example thereof is an oxidesemiconductor film in which no crystal part exists even in a microscopicregion, and the whole of the film is amorphous.

The microcrystalline oxide semiconductor film includes a microcrystal(also referred to as nanocrystal) with a size greater than or equal to 1nm and less than 10 nm, for example. Thus, the microcrystalline oxidesemiconductor film has a higher degree of atomic order than theamorphous oxide semiconductor film. Hence, the density of defect statesof the microcrystalline oxide semiconductor film is lower than that ofthe amorphous oxide semiconductor film.

The CAAC-OS film is one of oxide semiconductor films including aplurality of crystal parts, and most of the crystal parts each fitinside a cube whose one side is less than 100 nm. Thus, there is a casewhere a crystal part included in the CAAC-OS film fits inside a cubewhose one side is less than 10 nm, less than 5 nm, or less than 3 nm.The density of defect states of the CAAC-OS film is lower than that ofthe microcrystalline oxide semiconductor film. In a transmissionelectron microscope (TEM) image of the CAAC-OS film, a boundary betweencrystal parts, that is, a grain boundary is not clearly observed. Thus,in the CAAC-OS film, a reduction in electron mobility due to the grainboundary is less likely to occur.

According to the TEM image of the CAAC-OS film observed in a directionsubstantially parallel to a sample surface (cross-sectional TEM image),metal atoms are arranged in a layered manner in the crystal parts. Eachmetal atom layer has a morphology reflected by a surface over which theCAAC-OS film is formed (hereinafter, a surface over which the CAAC-OSfilm is formed is referred to as a formation surface) or a top surfaceof the CAAC-OS film, and is arranged in parallel to the formationsurface or the top surface of the CAAC-OS film.

In this specification, a term “parallel” indicates that the angle formedbetween two straight lines is greater than or equal to −10° and lessthan or equal to 10°, and accordingly also includes the case where theangle is greater than or equal to −5° and less than or equal to 5°. Inaddition, a term “perpendicular” indicates that the angle formed betweentwo straight lines is greater than or equal to 80° and less than orequal to 100°, and accordingly includes the case where the angle isgreater than or equal to 85° and less than or equal to 95°.

On the other hand, according to the TEM image of the CAAC-OS filmobserved in a direction substantially perpendicular to the samplesurface (plan-view TEM image), metal atoms are arranged in a triangularor hexagonal configuration in the crystal parts. However, there is noregularity of arrangement of metal atoms between different crystalparts.

From the results of the cross-sectional TEM image and the plan-view TEMimage, alignment is found in the crystal parts in the CAAC-OS film.

A CAAC-OS film is subjected to structural analysis with an X-raydiffraction (XRD) apparatus. For example, when the CAAC-OS filmincluding an InGaZnO₄ crystal is analyzed by an out-of-plane method, apeak appears frequently when the diffraction angle (2θ) is around 31°.This peak is derived from the (009) plane of the InGaZnO₄ crystal, whichindicates that crystals in the CAAC-OS film have c-axis alignment, andthat the c-axes are aligned in a direction substantially perpendicularto the formation surface or the top surface of the CAAC-OS film.

On the other hand, when the CAAC-OS film is analyzed by an in-planemethod in which an X-ray enters a sample in a direction substantiallyperpendicular to the c-axis, a peak appears frequently when 2θ is around56°. This peak is derived from the (110) plane of the InGaZnO₄ crystal.Here, analysis (φ scan) is performed under conditions where the sampleis rotated around a normal vector of a sample surface as an axis (φaxis) with 2θ fixed at around 56°. In the case where the sample is asingle-crystal oxide semiconductor film of InGaZnO₄, six peaks appear.The six peaks are derived from crystal planes equivalent to the (110)plane. On the other hand, in the case of a CAAC-OS film, a peak is notclearly observed even when φ scan is performed with 2θ fixed at around56°.

According to the above results, in the CAAC-OS film having c-axisalignment, while the directions of a-axes and b-axes are differentbetween crystal parts, the c-axes are aligned in a direction parallel toa normal vector of a formation surface or a normal vector of a topsurface. Thus, each metal atom layer arranged in a layered mannerobserved in the cross-sectional TEM image corresponds to a planeparallel to the a-b plane of the crystal.

Note that the crystal part is formed concurrently with deposition of theCAAC-OS film or is formed through crystallization treatment such as heattreatment. As described above, the c-axis of the crystal is aligned in adirection parallel to a normal vector of a formation surface or a normalvector of a top surface. Thus, for example, in the case where a shape ofthe CAAC-OS film is changed by etching or the like, the c-axis might notbe necessarily parallel to a normal vector of a formation surface or anormal vector of a top surface of the CAAC-OS film.

Further, the degree of crystallinity in the CAAC-OS film is notnecessarily uniform. For example, in the case where crystal growthleading to the CAAC-OS film occurs from the vicinity of the top surfaceof the film, the degree of the crystallinity in the vicinity of the topsurface is higher than that in the vicinity of the formation surface insome cases. Further, when an impurity is added to the CAAC-OS film, thecrystallinity in a region to which the impurity is added is changed, andthe degree of crystallinity in the CAAC-OS film varies depending onregions.

Note that when the CAAC-OS film with an InGaZnO₄ crystal is analyzed byan out-of-plane method, a peak of 2θ may also be observed at around 36°,in addition to the peak of 2θ at around 31°. The peak of 2θ at around36° indicates that a crystal having no c-axis alignment is included inpart of the CAAC-OS film. It is preferable that in the CAAC-OS film, apeak of 2θ appear at around 31° and a peak of 2θ do not appear at around36°.

In a transistor using the CAAC-OS film, a change in electricalcharacteristics due to irradiation with visible light or ultravioletlight is small. Thus, the transistor has high reliability.

Note that an oxide semiconductor film may be a stacked film includingtwo or more films of an amorphous oxide semiconductor film, amicrocrystalline oxide semiconductor film, and a CAAC-OS film, forexample.

For the deposition of the CAAC-OS film, the following conditions arepreferably used.

By reducing the amount of impurities entering the CAAC-OS film duringthe deposition, the crystal state can be prevented from being broken bythe impurities. For example, the concentration of impurities (e.g.,hydrogen, water, carbon dioxide, and nitrogen) that exist in thetreatment chamber may be reduced. Furthermore, the concentration ofimpurities in a deposition gas may be reduced. Specifically, adeposition gas whose dew point is −80° C. or lower, preferably −100° C.or lower is used.

By increasing the substrate heating temperature during the deposition,migration of a sputtered particle occurs after the sputtered particlereaches the substrate. Specifically, the substrate heating temperatureduring the deposition is higher than or equal to 100° C. and lower thanor equal to 740° C., preferably higher than or equal to 200° C. andlower than or equal to 500° C. When the substrate heating temperatureduring the deposition is increased and flat-plate-like or pellet-likesputtered particles reach the substrate, migration occurs on thesubstrate, so that a flat plane of each sputtered particle is attachedto the substrate.

Furthermore, it is preferable that the proportion of oxygen in thedeposition gas be increased and the power be optimized in order toreduce plasma damage at the deposition. The proportion of oxygen in thedeposition gas is higher than or equal to 30 vol %, preferably 100 vol%.

As an example of the target, an In—Ga—Zn-based oxide target is describedbelow.

The In—Ga—Zn-based oxide target, which is polycrystalline, is made bymixing InO_(X) powder, GaO_(Y) powder, and ZnO_(Z) powder in apredetermined molar ratio, applying pressure, and performing heattreatment at a temperature higher than or equal to 1000° C. to and lowerthan or equal to 1500° C. Note that X, Y, and Z are each a givenpositive number. Here, the predetermined molar ratio of InO_(X) powderto GaO_(Y) powder and ZnO_(Z) powder is, for example, 2:2:1, 8:4:3,3:1:1, 1:1:1, 4:2:3, or 3:1:2. The kinds of powder and the molar ratiofor mixing powder may be determined as appropriate depending on thedesired target.

An alkali metal is not an element included in an oxide semiconductor andthus is an impurity. Also, alkaline earth metal is an impurity in thecase where the alkaline earth metal is not a component of the oxidesemiconductor. Alkali metal, in particular, Na becomes Na⁺ when aninsulating film in contact with the oxide semiconductor film is an oxideand Na diffuses into the insulating film. Further, in the oxidesemiconductor film, Na cuts or enters a bond between metal and oxygenwhich are components of the oxide semiconductor. As a result, theelectrical characteristics of the transistor deteriorate, for example,the transistor is placed in a normally-on state because of a negativeshift of the threshold voltage or the mobility is decreased. Inaddition, the characteristics of transistors vary. Specifically, the Naconcentration measured by secondary ion mass spectrometry is preferably5×10¹⁶/cm³ or lower, further preferably 1×10¹⁶/cm³ or lower, stillfurther preferably 1×10¹⁵/cm³ or lower. Similarly, the measured Liconcentration is preferably 5×10¹⁵/cm³ or lower, further preferably1×10¹⁵/cm³ or lower. Similarly, the measured K concentration ispreferably 5×10¹⁵/cm³ or lower, further preferably 1×10¹⁵/cm³ or lower.

In the case where metal oxide containing indium is used, silicon orcarbon having higher bond energy with oxygen than indium might cut thebond between indium and oxygen, so that an oxygen vacancy may be formed.Accordingly, when silicon or carbon is contained in the oxidesemiconductor film, the electrical characteristics of the transistor arelikely to deteriorate as in the case of using alkali metal or alkalineearth metal. Thus, the concentrations of silicon and carbon in the oxidesemiconductor film are preferably low. Specifically, the C concentrationor the Si concentration measured by secondary ion mass spectrometry ispreferably less than or equal to 1×10¹⁸/cm³. In this case, thedeterioration of the electrical characteristics of the transistor can beprevented, so that the reliability of a semiconductor device can beimproved.

A metal in the source and drain electrodes might extract oxygen from theoxide semiconductor film depending on a conductive material used for thesource and drain electrodes. In such a case, regions of the oxidesemiconductor film in contact with the source and drain electrodesbecome n-type regions because of the formation of an oxygen vacancy.

The n-type regions serve as source and drain regions, resulting in adecrease in the contact resistance between the oxide semiconductor filmand the source electrode or the drain electrode. Accordingly, theformation of the n-type regions increases the mobility and the on-statecurrent of the transistor, which achieves high-speed operation of asemiconductor device using the transistor.

Note that the extraction of oxygen by a metal in the source and drainelectrodes is probably caused when the source and drain electrodes areformed by a sputtering method or when heat treatment is performed afterthe formation of the source and drain electrodes.

The n-type regions are more likely to be formed when the source anddrain electrodes are formed using a conductive material that is easilybonded to oxygen. Examples of such a conductive material include Al, Cr,Cu, Ta, Ti, Mo, and W.

The oxide semiconductor film is not limited to a single-layer metaloxide film and may have a stacked structure of a plurality of metaloxide films. In a semiconductor film in which first to third metal oxidefilms are stacked in this order, for example, the first metal oxide filmand the third metal oxide film are each an oxide film that contains atleast one of the metal elements contained in the second metal oxide filmand whose conduction band minimum is closer to the vacuum level thanthat of the second metal oxide film by higher than or equal to 0.05 eV,0.07 eV, 0.1 eV, or 0.15 eV and lower than or equal to 2 eV, 1 eV, 0.5eV, or 0.4 eV. Furthermore, the second metal oxide film preferablycontains at least indium, in which case the carrier mobility of thesecond metal oxide film is increased.

In the transistor including the above oxide semiconductor film, when avoltage is applied to the gate electrode so that an electric field isapplied to the semiconductor film, a channel region is formed in thesecond metal oxide film whose conduction band minimum is small in thesemiconductor film. That is, since the third metal oxide film isprovided between the second metal oxide film and the gate insulatingfilm, a channel region can be formed in the second metal oxide film thatis insulated from the gate insulating film.

Since the third metal oxide film contains at least one of the metalelements contained in the second metal oxide film, interface scatteringis unlikely to occur at the interface between the second metal oxidefilm and the third metal oxide film. Thus, the movement of carriers isunlikely to be inhibited at the interface, resulting in an increase inthe field-effect mobility of the transistor.

When an interface level is formed at the interface between the secondmetal oxide film and the first metal oxide film, a channel region isformed also in the vicinity of the interface, which causes a change inthe threshold voltage of the transistor. However, since the first metaloxide film contains at least one of the metal elements contained in thesecond metal oxide film, an interface level is unlikely to be formed atthe interface between the second metal oxide film and the first metaloxide film. Accordingly, the above structure can reduce variations inthe electrical characteristics of the transistor, such as the thresholdvoltage.

Further, a plurality of metal oxide films are preferably stacked so thatan interface level that inhibits carrier flow is not formed at theinterface between the metal oxide films due to an impurity existingbetween the metal oxide films. This is because when an impurity existsbetween the stacked metal oxide films, the continuity of the conductionband minimum between the metal oxide films is lost, and carriers aretrapped or disappear by recombination in the vicinity of the interface.By reducing an impurity existing between the films, a continuousjunction (here, particularly a U-shape well structure whose conductionband minimum is changed continuously between the films) is formed moreeasily than the case of merely stacking a plurality of metal oxide filmsthat contain at least one common metal as a main component.

In order to form such a continuous junction, the films need to bestacked successively without being exposed to the air by using amulti-chamber deposition system (sputtering system) provided with a loadlock chamber. Each chamber of the sputtering apparatus is preferablyevacuated to a high vacuum (to approximately 5×10⁻⁷ Pa to 1×10⁻⁴ Pa) byan adsorption vacuum pump such as a cryopump so that water and the likeacting as impurities for the oxide semiconductor are removed as much aspossible. Alternatively, a turbo molecular pump and a cold trap arepreferably used in combination to prevent backflow of gas into thechamber through an evacuation system.

To obtain a highly purified intrinsic oxide semiconductor, not only highvacuum evacuation of the chambers but also high purification of a gasused in the sputtering is important. When an oxygen gas or an argon gasused as the sputtering gas has a dew point of −40° C. or lower,preferably −80° C. or lower, further preferably −100° C. or lower and ishighly purified, moisture and the like can be prevented from enteringthe oxide semiconductor film as much as possible. Specifically, when thesecond metal oxide film is an In-M-Zn oxide film (M is Ga, Y, Zr, La,Ce, or Nd) and a target having an atomic ratio of metal elements ofIn:M:Zn=x₁:y₁:z₁ is used to form the second metal oxide film, x₁/y₁ranges preferably from ⅓ to 6, further preferably from 1 to 6, and z₁/y₁ranges preferably from ⅓ to 6, further preferably from 1 to 6. Note thatwhen z₁/y₁ ranges from 1 to 6, a CAAC-OS film is likely to be formed asthe second metal oxide film. Typical examples of the atomic ratio of themetal elements in the target are In:M:Zn=1:1:1 and In:M:Zn=3:1:2.

Specifically, when the first and third metal oxide films are each anIn—M—Zn oxide film (M is Ga, Y, Zr, La, Ce, or Nd) and a target used fordepositing the first and third metal oxide films has an atomic ratio ofmetal elements of In:M:Zn=x₂:y₂:z₂, x₂/y₂<x₁/y₁ is satisfied and z₂/y₂ranges preferably from ⅓ to 6, further preferably from 1 to 6. Note thatwhen z₂/y₂ ranges from 1 to 6, CAAC-OS films are likely to be formed asthe first and third metal oxide films. Typical examples of the atomicratio of the metal elements of the target are In:M:Zn=1:3:2,In:M:Zn=1:3:4, In:M:Zn=1:3:6, In:M:Zn=1:3:8, and the like.

The first and third metal oxide films each have a thickness of 3 nm to100 nm, preferably 3 nm to 50 nm. The second metal oxide film has athickness of 3 nm to 200 nm, preferably 3 nm to 100 nm, furtherpreferably 3 nm to 50 nm.

In the three-layer semiconductor film, each of the first to third metaloxide films can be amorphous or crystalline. Note that the second metaloxide film in which a channel region is formed is preferablycrystalline, in which case the transistor can have stable electricalcharacteristics.

Note that a “channel formation region” refers to a region of asemiconductor film of a transistor that overlaps with a gate electrodeand is located between a source electrode and a drain electrode.Further, a “channel region” refers to a region through which currentmainly flows in the channel formation region.

For example, when an In—Ga—Zn-based oxide film formed by a sputteringmethod is used as each of the first and third metal oxide films, thefirst and third metal oxide films can be deposited with use of anIn—Ga—Zn-based oxide target containing In, Ga, and Zn in an atomic ratioof 1:3:2. The deposition conditions can be as follows, for example: anargon gas (flow rate: 30 sccm) and an oxygen gas (flow rate: 15 sccm)are used as the deposition gas; the pressure is 0.4 Pa; the substratetemperature is 200° C.; and the DC power is 0.5 kW.

Further, when the second metal oxide film is a CAAC-OS film, the secondmetal oxide film is preferably deposited with use of a polycrystallineIn—Ga—Zn-based oxide target containing In, Ga, and Zn in an atomic ratioof 1:1:1. The deposition conditions can be as follows, for example: anargon gas (flow rate: 30 sccm) and an oxygen gas (flow rate: 15 sccm)are used as the deposition gas; the pressure is 0.4 Pa; the substratetemperature is 300° C.; and the DC power is 0.5 kW.

Note that the end portion of the semiconductor film included in thetransistor may be inclined or may be rounded.

Also in the case where a semiconductor film including stacked metaloxide films is used in the transistor, a region in contact with thesource electrode or the drain electrode can be an n-type region. Such astructure increases the mobility and the on-state current of thetransistor and achieves high-speed operation of a semiconductor deviceusing the transistor. Furthermore, when the semiconductor film includingthe stacked metal oxide films is used in the transistor, the n-typeregion particularly preferably reaches the second metal oxide film partof which is to be a channel region, because the mobility and theon-state current of the transistor are further increased andhigher-speed operation of the semiconductor device is achieved.

Structure Example of Electronic Device Using Semiconductor Device

The semiconductor device of one embodiment of the present invention canbe used for display devices, personal computers, or image reproducingdevices provided with recording media (typically, devices that includedisplays, and can reproduce the content of recording media such asdigital versatile discs (DVDs) and display the reproduced images). Inaddition, examples of electronic devices in which the semiconductordevice of one embodiment of the present invention can be used includecellular phones, game machines (including portable game machines),personal digital assistants, e-book readers, cameras such as videocameras and digital still cameras, goggle-type displays (head mounteddisplays), navigation systems, audio reproducing devices (e.g., caraudio systems and digital audio players), copiers, facsimiles, printers,multifunction printers, automated teller machines (ATMs), and vendingmachines. Specific examples of these electronic devices are illustratedin FIGS. 16A to 16F.

FIG. 16A illustrates a portable game machine, which includes a housing5001, a housing 5002, a display portion 5003, a display portion 5004, amicrophone 5005, speakers 5006, an operation key 5007, a stylus 5008,and the like. The semiconductor device of one embodiment of the presentinvention can be used for the display portion 5003, the display portion5004, or an integrated circuit in another portion. Note that althoughthe portable game machine in FIG. 16A has the two display portions 5003and 5004, the number of display portions included in the portable gamemachine is not limited thereto.

FIG. 16B illustrates a personal digital assistant, which includes afirst housing 5601, a second housing 5602, a first display portion 5603,a second display portion 5604, a joint 5605, an operation key 5606, andthe like. The first display portion 5603 is provided in the firsthousing 5601, and the second display portion 5604 is provided in thesecond housing 5602. The first housing 5601 and the second housing 5602are connected to each other with the joint 5605, and an angle betweenthe first housing 5601 and the second housing 5602 can be changed withthe joint 5605. Images on the first display portion 5603 may be switchedin accordance with the angle at the joint 5605 between the first housing5601 and the second housing 5602. The semiconductor device of oneembodiment of the present invention can be used for the first displayportion 5603, the second display portion 5604, or an integrated circuitin another portion.

FIG. 16C illustrates a laptop personal computer, which includes ahousing 5401, a display portion 5402, a keyboard 5403, a pointing device5404, and the like. The semiconductor device of one embodiment of thepresent invention can be used for the display portion 5402 or anintegrated circuit in another portion.

FIG. 16D illustrates a wristwatch, which includes a housing 5201, adisplay portion 5202, an operation button 5203, a bracelet 5204, and thelike. The semiconductor device of one embodiment of the presentinvention can be used for the display portion 5202 or an integratedcircuit in another portion.

FIG. 16E illustrates a video camera, which includes a first housing5801, a second housing 5802, a display portion 5803, operation keys5804, a lens 5805, a joint 5806, and the like. The operation keys 5804and the lens 5805 are provided in the first housing 5801, and thedisplay portion 5803 is provided in the second housing 5802. The firsthousing 5801 is connected to the second housing 5802 with the joint5806, and the angle between the first housing 5801 and the secondhousing 5802 can be changed at the joint 5806. Images on the displayportion 5803 may be switched in accordance with the angle at the joint5806 between the first housing 5801 and the second housing 5802. Thesemiconductor device of one embodiment of the present invention can beused for the display portion 5803 or an integrated circuit in anotherportion.

FIG. 16F illustrates a cellular phone. In the cellular phone, a displayportion 5902, a microphone 5907, a speaker 5904, a camera 5903, anexternal connection portion 5906, and an operation button 5905 areprovided in a housing 5901. The semiconductor device of one embodimentof the present invention can be used for the display portion 5902 or anintegrated circuit in another portion. When the semiconductor device ofone embodiment of the present invention is provided over a flexiblesubstrate, the semiconductor device can be used as the display portion5902 having a curved surface, as illustrated in FIG. 16F.

This application is based on Japanese Patent Application serial No.2013-144190 filed with Japan Patent Office on Jul. 10, 2013, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A semiconductor device comprising: a circuit towhich an input signal is supplied; a first transistor; a secondtransistor; a first wiring to which a first potential is supplied, thefirst wiring electrically connected to a gate of the first transistorand a gate of the second transistor through the circuit; a second wiringto which a second potential is supplied, the second wiring electricallyconnected to one of a source and a drain of the first transistor; athird wiring to which a third potential is supplied, the third wiringelectrically connected to the gate of the first transistor and the gateof the second transistor through the circuit; and a fourth wiring towhich a first clock signal is supplied, the fourth wiring electricallyconnected to one of a source and a drain of the second transistor,wherein the other of the source and the drain of the first transistor iselectrically connected to the other of the source and the drain of thesecond transistor, wherein the circuit is configured to controlelectrical connections between the gates of the first and secondtransistors and the first and third wirings in accordance with the inputsignal and a second clock signal supplied to the circuit, wherein thefirst clock signal alternates the second potential and a fourthpotential, and the second clock signal alternates the first potentialand the third potential, wherein the second potential is higher than thefirst potential, wherein the third potential is higher than the secondpotential, wherein the fourth potential is equal to or higher than thethird potential, and wherein the first transistor and the secondtransistor have the same conductivity type.
 2. A semiconductor deviceaccording to claim 1, comprising: a fifth wiring to which the secondclock signal is supplied, wherein the fifth wiring is electricallyconnected to the circuit.
 3. A semiconductor device according to claim1, wherein the first transistor and the second transistor each comprisea channel formation region in an oxide semiconductor film.
 4. Asemiconductor device according to claim 3, wherein the oxidesemiconductor film comprises In, Ga, and Zn.
 5. A semiconductor deviceaccording to claim 1, wherein the circuit comprises transistors, andwherein the transistors each include a channel formation region in anoxide semiconductor film.
 6. A semiconductor device according to claim5, wherein a channel width of the first transistor is larger than achannel width of the transistor of the circuit.
 7. A semiconductordevice according to claim 1, comprising: a pixel portion; and a drivercircuit comprising the circuit, the first transistor, and the secondtransistor.
 8. A semiconductor device comprising: a first transistor; asecond transistor; and a circuit electrically connected to a gate of thefirst transistor and a gate of the second transistor, wherein one of asource and a drain of the first transistor is electrically connected toone of a source and a drain of the second transistor, wherein a firstpotential and a third potential are supplied to the circuit through afirst wiring and a second wiring, respectively, wherein a secondpotential is supplied to the other of the source and the drain of thefirst transistor, wherein a first clock signal is supplied to the otherof the source and the drain of the second transistor, and a second clocksignal is supplied to the circuit, wherein the circuit is configured tocontrol electrical connections between the gates of the first and secondtransistors and the first and second wirings, wherein the first clocksignal alternates the second potential and a fourth potential, and thesecond clock signal alternates the first potential and the thirdpotential, wherein the fourth potential is equal to or higher than thethird potential, wherein the third potential is higher than the secondpotential, and wherein the second potential is higher than the firstpotential.
 9. A semiconductor device according to claim 8, wherein thefirst transistor and the second transistor have the same conductivitytype.
 10. A semiconductor device according to claim 8, wherein the firsttransistor and the second transistor each comprise a channel formationregion in an oxide semiconductor film.
 11. A semiconductor deviceaccording to claim 10, wherein the oxide semiconductor film comprisesIn, Ga, and Zn.
 12. A semiconductor device according to claim 8, whereinthe circuit comprises transistors, and wherein the transistors eachinclude a channel formation region in an oxide semiconductor film.
 13. Asemiconductor device according to claim 12, wherein a channel width ofthe first transistor is larger than a channel width of the transistor ofthe circuit.
 14. A semiconductor device according to claim 8,comprising: a pixel portion; and a driver circuit comprising thecircuit, the first transistor, and the second transistor.
 15. Asemiconductor device comprising: a shift register comprising a firstcircuit and a second circuit electrically connected to each other, thefirst circuit and the second circuit each comprising: a circuit to whichan input signal is supplied; a first transistor, a gate of the firsttransistor electrically connected to the circuit; a second transistor, agate of the second transistor electrically connected to the circuit;wherein one of a source and a drain of the first transistor iselectrically connected to one of a source and a drain of the secondtransistor, wherein a first potential is supplied to the circuit througha first wiring, wherein a second potential is supplied to the other ofthe source and the drain of the first transistor through a secondwiring, and the second potential is higher than the first potential,wherein a third potential is supplied to the circuit through a thirdwiring, and the third potential is higher than the second potential,wherein the circuit is configured to control electrical connectionsbetween the gates of the first and second transistors and the first andthird wirings, wherein a first clock signal is supplied to the other ofthe source and the drain of the second transistor of the first circuit,wherein a second clock signal is supplied to the circuit of the firstcircuit, wherein a third clock signal is supplied to the other of thesource and the drain of the second transistor of the second circuit,wherein a fourth clock signal is supplied to the circuit of the secondcircuit, wherein the first clock signal and the third clock signal eachalternate the second potential and a fourth potential, and the secondclock signal and the fourth clock signal each alternate the firstpotential and the third potential, wherein the fourth potential is equalto or higher than the third potential, and wherein an output signal ofthe first circuit is supplied to the circuit of the second circuit asthe input signal.
 16. A semiconductor device according to claim 15,wherein the first transistor and the second transistor have the sameconductivity type.
 17. A semiconductor device according to claim 15,wherein the first transistor and the second transistor each comprise achannel formation region in an oxide semiconductor film.
 18. Asemiconductor device according to claim 17, wherein the oxidesemiconductor film comprises In, Ga, and Zn.
 19. A semiconductor deviceaccording to claim 15, wherein the circuit comprises transistors, andwherein the transistors each include a channel formation region in anoxide semiconductor film.
 20. A semiconductor device according to claim19, wherein a channel width of the first transistor is larger than achannel width of the transistor of the circuit.
 21. A semiconductordevice according to claim 15, comprising: a pixel portion; and a drivercircuit comprising the shift register.